# XPS Foam For Foam Core Builds



## SurfDuffer (Dec 23, 2020)

Been looking at some of the foam core builds here and was wondering if anyone has done one using the blue, green or pink XPS foam you can buy at any home improvement store? It is completely compatible with Epoxy. There are some tricks to working with it but it would seem to me to be useful for one of these builds.


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## el9surf (Dec 23, 2008)

SurfDuffer said:


> Been looking at some of the foam core builds here and was wondering if anyone has done one using the blue, green or pink XPS foam you can buy at any home improvement store? It is completely compatible with Epoxy. There are some tricks to working with it but it would seem to me to be useful for one of these builds.


I know years ago many guys tried to make surfboards out of the same foam as a less expensive diy alternative. In the heat the foam would off gas and the glass would delam. Not sure about the epoxy route.


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## Fishtex (May 10, 2014)

Other than the fact it has no peel strength, and is not engineered to be a structural core it should be fine, for a cooler for your patio. Poly resin melts it though....


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## DuckNut (Apr 3, 2009)

That type of foam has no place on a boat other than a cooler. There is very little peel, shear and compression strength and is not suitable for any structural use and every place on a boat is structural - other than a cooler which sits on the sole of the skiff.


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## ShugC (Apr 23, 2016)

DuckNut said:


> That type of foam has no place on a boat other than a cooler. There is very little peel, shear and compression strength and is not suitable for any structural use and every place on a boat is structural - other than a cooler which sits on the sole of the skiff.


 I beg to differ you have never seen me sit on a cooler. it better be structural also or the beer is screwed.


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## DuckNut (Apr 3, 2009)

ShugC said:


> I beg to differ you have never seen me sit on a cooler. it better be structural also or the beer is screwed.


I don't care who you are - that is funny shit right there.


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## SurfDuffer (Dec 23, 2020)

el9surf said:


> I know years ago many guys tried to make surfboards out of the same foam as a less expensive diy alternative. In the heat the foam would off gas and the glass would delam. Not sure about the epoxy route.


Actually this is why I bring it up. With a few design accommodations the delam issue is not an issue. In surfboards the delam issue was mostly due to very thin skins and people leaving the surfboard in a blazing hot car and builders who did not provide vents in the boards, XPS is 100% compatible with epoxy. Structurally it is every bit as strong as some foams used in boat building applications. In fact it is more dense and harder to compress than some foams used in boat building. Obviously you're not going to use it for transoms but I could easily see it in coring hull sides, bulkheads, floors and such in micro-skiffs. Let's not forget that the foam is not where the strength resides in composite construction. The strength is in the skins. The foam is the web of the I-Beam.


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## DuckNut (Apr 3, 2009)

Seems like your research has led you to the promise land where no other builder has been able to go. If it is just as good as the tried and true foam for 1/10th the price - GO FOR IT.

Might want to keep it a secret - before long there will be a run on HD insulation from the builders.


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## WC53 (Dec 2, 2015)

Outgassing separates the skins? EPS not XPS is bonded to thin divinycell and then glass in many sailboards to get a tougher skin and keep weight down. There might be a reason for those higher dollar products, sheet strength, compression, sandability, water absorption, adhesion.... ymmv. The interweb thingy is full of XPS failure threads.


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## SurfDuffer (Dec 23, 2020)

I'm a longtime surfboard builder. I come to this with 30+ years of working with fiberglass, foams and resins. I'm not confusing EPS and XPS and I am well aware of the use of divinicel in eps sandwich construction. Just curious if anyone here has built a microskiff using it.


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## DuckNut (Apr 3, 2009)

If it doesn't work for surfboards - it surely won't work for a boat.

Like I said above...use your 30 years of experience and show the boat building industry they have been using the wrong product.


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## firecat1981 (Nov 27, 2007)

Go for it, and please document every step here, the good and the ugly. 

Where did you get the info that xps foam is as strong as typical coring? I have some here, along with carbon core, they are not even close. Surfboards can be made out of anything that floats because they don't face near the pressures of a typical planing hull.


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## birdyshooter (Sep 17, 2019)

Balsa for life!!!


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## SurfDuffer (Dec 23, 2020)

DuckNut said:


> If it doesn't work for surfboards - it surely won't work for a boat.
> 
> Like I said above...use your 30 years of experience and show the boat building industry they have been using the wrong product.


Who said it doesn't work for surfboards? It does work for surfboards when done right. Early on some failed with it because they didn't know how to work with it and the surfers who left those early boards in their cars on a hot summer day had their boards blow up like a balloon because they were not vented. Here is the link to the biggest manufacturer building boards with this foam and process. Buy Custom Epoxy Surfboards Online

I did not say or suggest that the boat building industry is using the "wrong product". Saying this is the "wrong product" for a home built microskiff seems ignorant if you've never used this foam in any way. For a home built microskiff I view this as a viable and readily available alternative to more expensive products.



firecat1981 said:


> Go for it, and please document every step here, the good and the ugly.
> 
> Where did you get the info that xps foam is as strong as typical coring? I have some here, along with carbon core, they are not even close. Surfboards can be made out of anything that floats because they don't face near the pressures of a typical planing hull.


The foams used in boat building come in all sorts of densities and each have their place. Words mean things. I said quote: "Structurally it is every bit as strong as some foams used in boat building applications." The word SOME was used for a reason. I was not suggesting it was as strong as carbon core for christ sakes but is more dense and stronger than some of the lighter foams used in boat building.

I'll add that pound for pound a surfboard faces greater stresses than a microskiff fishing sheltered waters will ever face.



birdyshooter said:


> Balsa for life!!!


My nearly 40 year old Mako from the factory has balsa used in parts of the floor and console along with a very low density polyurethane foam sheet material that was used to core the forward deck. XPS foam has similar characteristics to balsa and is impervious to water yet I'm told here its not possible. Honestly don't think some of the people who have replied even know what XPS is and are confusing it with EPS. Specifically our friends who want to mock with references to coolers.

We are talking micrskiffs here not offshore battlewagons or high horsepower goesfast boats. I'm here because of my interest in using my surfboard building skills to make a small skiff. When I do there will likely be several different foams in the hull. No doubt it will be documented but it likely won't be posted where closed minded nitpickers will tell me it can't be done.

Sorry if my thinking outside the box and asking a simple question brought out the inner troll in some of you. Cheers.


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## firecat1981 (Nov 27, 2007)

SurfDuffer said:


> The foams used in boat building come in all sorts of densities and each have their place. Words mean things. I said quote: "Structurally it is every bit as strong as some foams used in boat building applications." The word SOME was used for a reason. I was not suggesting it was as strong as carbon core for christ sakes but is more dense and stronger than some of the lighter foams used in boat building.
> 
> I'll add that pound for pound a surfboard faces greater stresses than a microskiff fishing sheltered waters will ever face.


I'm calling total BS on this. I think you are confusing density with structural strength. The 2 are not the same and there is a lot more that goes into it. Density only adds, somewhat, to compressive strength. It does little for flexibility, peel strength, torsional strength, frangibility .... you know, all the things you actually need for a boat core. 
Post up where you found your data that it's as strong as "SOME" of the other foams so we can review your data.

Having surfed, owned boards, and built several boats I would agree that a surfboard faces greater pressure, IF they were both static in calm waters. As soon as you power a hull to plane it faces many times greater force then the board will ever see. There is no apples to apples comparison here.

Typical internet defensive measure is to call someone a troll for pointing things out you don't agree with. It's lame. You can go back through some of my old builds and see I actually did layups with xps for fun to compare it. It failed easily as it has no real peel strength.


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## birdyshooter (Sep 17, 2019)

@SurfDuffer , give it a try. Perforate the hell out of it with a wallpaper tool and I wouldn't use anything other than epoxy.


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## SurfDuffer (Dec 23, 2020)

birdyshooter said:


> @SurfDuffer , give it a try. Perforate the hell out of it with a wallpaper tool and I wouldn't use anything other than epoxy.


Good on you for knowing about the wallpapering tool trick and knowing that polyurethane resin will melt it.


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## ShugC (Apr 23, 2016)

Ok well I am now completely confused. can I build in foam from home depot or do I really need to spend the extra cash on carbon core? if I use home depot foam I need to change from poly resin to epoxy? and do I need to change the laminate schedule for my skiff if I go with the Home Depot foam?


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## birdyshooter (Sep 17, 2019)

SurfDuffer said:


> Good on you for knowing about the wallpapering tool trick and knowing that polyurethane resin will melt it.


I was in the same boat as you a few years ago doing research on core materials. I decided to just keep my hull solid fiberglass. Adhesion with any synthetic is going to be the biggest issue from what I found. I'm still a big proponent of balsa for adhesive qualities. As long as you know how to keep it water tight, it is a phenomenal core choice.


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## SurfDuffer (Dec 23, 2020)

ShugC said:


> Ok well I am now completely confused. can I build in foam from home depot or do I really need to spend the extra cash on carbon core? if I use home depot foam I need to change from poly resin to epoxy? and do I need to change the laminate schedule for my skiff if I go with the Home Depot foam?


This sort of foam is not compatible with polyester resin. You have to use epoxy.


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## ShugC (Apr 23, 2016)

but otherwise I am good to go?


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## JC Designs (Apr 5, 2020)

ShugC said:


> Ok well I am now completely confused. can I build in foam from home depot or do I really need to spend the extra cash on carbon core? if I use home depot foam I need to change from poly resin to epoxy? and do I need to change the laminate schedule for my skiff if I go with the Home Depot foam?


SHUG, take it from the guy that did your lam schedule... use what I told ya and don’t deviate!

And for God’s sake, don’t anybody try to build a foam cored powered vessel from XPS or EPS foam!!! Damn it, why is it so hard to understand that XPS and EPS are an insulation not a structural product. Look at sheer/peel/moe/friability/melt point/density/etc... and it is clear that the structural grade marine and aerospace foams are what should be used. The reason different densities, thicknesses, strength foams are used in many production boats simply comes down to $ aka best bang for the buck. And in the case of lightweight and high performance vessels it comes down to engineering the lightest yet strongest vessel for a given weight. I.O.Ws, use the righy core in the right spot to achieve the designs goals and strength needs.


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## birdyshooter (Sep 17, 2019)

I'm going to try and be a completely objective asshole here. There has been a new wonder core just like there is a new slice free driver every year since the dawn of time. Neither is promising, but with proper fundamentals things can be achieved. Know your materials and their limitations and expect their pluses and minuses.


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## DuckNut (Apr 3, 2009)

What is really going on here?

You are the only person who is saying it can be a viable product and everyone else says otherwise (including an entire industry).

Are you a shill for an insulation company?

If not, when will you build it?

You seem to have it all figured out - get on with it.


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## SurfDuffer (Dec 23, 2020)

At the end of the day, the foam just provides the spacer between two skins. The strength is in the skins not the foam. The only real exception would be the transom where you are hanging your motor. If your skins and your foam are in the correct proportion to the loads they will experience there is no problem. That is the essence of composites. Its intriguing to read the posts from people who insist things can't be done or must be done their way and the conviction with which they insist.

I just want to check. We are talking microskiffs here.......not offshore battlewagons smashing through inlets. Just want to make sure we are on the same page here........casue I'm thinking 14 foot one or two man 20hp microskiff on calm protected waters. 


▼▼▼ *not this* ▼▼▼ ▼▼▼ *not this* ▼▼▼ ▼▼▼ *not this* ▼▼▼ ▼▼▼ *not this* ▼▼▼


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## firecat1981 (Nov 27, 2007)

SurfDuffer said:


> At the end of the day, the foam just provides the spacer between two skins. The strength is in the skins not the foam. The only real exception would be the transom where you are hanging your motor. If your skins and your foam are in the correct proportion to the loads they will experience there is no problem. That is the essence of composites. Its intriguing to read the posts from people who insist things can't be done or must be done their way and the conviction with which they insist.


This is wrong, completely. The definition of a composite is made up of various parts or elements. Meaning they need to work together. The core/foam is not just a spacer between the skins, it becomes a structural member helping to support the whole. Like I said, I've done test layups with xps, I'm not just talking here. Could you make the skins thick enough so the foam core doesn't matter, yes, it's called making a mold. At which point it's better to remove the foam completely after the layup, so it's no longer a core element. Using foam as a form or mold is NOT composite building by definition because it is not working to help each other.



SurfDuffer said:


> I just want to check. We are talking microskiffs here.......not offshore battlewagons smashing through inlets. Just want to make sure we are on the same page here........casue I'm thinking 14 foot one or two man 20hp microskiff on calm protected waters.


Yes we are talking lightweight skiffs here, which makes the coring more important then your "battlewagons". In an offshore boat you need thick glass to support the amount of distortion that can occure from the pressures exerted. A 1/2" thick skin of glass is not just 4x stronger then 1/8", the strength grows exponentially as the thickness increases. Heavy weight is an advantage offshore, but a major disadvantage in the shallows. If both hulls were running the same smooth waters then the glass could be thinner, but you need to account for the distortion that occurs when running on plane to the hull bottom. Your XPS foam and no peel strength and low frangibility. This means the distortion over time will make it fail and crumble. There have been many builders over the years who tried inferior products and the results showed. Rippled skins, seperate skins, cracks in the glass from lack of support.....
You are trying to equate skiffs to surfboards and I get it. A surfboard has so little pressure on the bottom that you can use just the foam with no skin. Any of us who remember spring break in cocoa beach or Lauderdale, remember that was the thing to do for visitors. They would buy Styrofoam surfboards or boogieboards, use them for the weekend and then toss them in the dumpster. Will they last, nope, that's why a good board costs so much, but they did function. You couldn't do that on a skiff, the amount of pressure per square inch increases dramatically with speed even over smooth waters. Thin fiberglass skins need proper and durable backing to deal with this. Thicker skins do not, but at that point you no longer have a lightweight skiff so what was the point?

I get it, look through my builds and you will see I have tried to reinvent the wheel from time to time. In most cases it seems a wheel rolls best when in the tried and true round shape. You have guys here advising you that have built boats personally, and professionally, using various composites. If you don't believe us then go do some test samples and see for yourself like some of us have. I'll predict your results based on my own. You will find out that using an inferior core means thicker glass, your boat will end up heavier and more expensive then if you spent the money on a proper core.


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## JC Designs (Apr 5, 2020)

Ok, I’ve had enough! I will personally grab a sheet of 3/4XPS foam and do a side x side using carbon core 5lb pe foam “most economical core foam available” and do some flex/peel/MOE/and the good old fashioned BFH tests provided the XPS makes it past the flex test! Will laminate both sides with [email protected] 0/90 & 1 @1708 45/45 resin will be epoxy! Maybe I too can save a small fortune on my personal stuff since I would never dream of endangering a client’s life or his/her loved ones using F’ing insulation for a structural core in a vessel that takes much more abuse than the so called battle wagons! Wash board ripples create a harmonic effect that will straight up destroy boats over time if they are not engineered and built properly! Stay tuned!😉

PS, stop arguing about it! Many of us have tried alternatives and all samples have failed miserably, PERIOD! PS, you are arguing with some actual professionals in the industry also so perhaps you should take their advise and pull your head from your dark spot for just a second! And surfboards ain’t skiffs and while they take a pounding, the forces generated are not comparable to what a skiff sees! Apples/oranges my friend! Carry on and build it if you want, I’d like to see it!


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## DuckNut (Apr 3, 2009)

JC Designs said:


> Ok, I’ve had enough! I will personally grab a sheet of 3/4XPS foam and do a side x side using carbon core 5lb pe foam “most economical core foam available” and do some flex/peel/MOE/and the good old fashioned BFH tests provided the XPS makes it past the flex test! Will laminate both sides with [email protected] 0/90 & 1 @1708 45/45 resin will be epoxy! Maybe I too can save a small fortune on my personal stuff since I would never dream of endangering a client’s life or his/her loved ones using F’ing insulation for a structural core in a vessel that takes much more abuse than the so called battle wagons! Wash board ripples create a harmonic effect that will straight up destroy boats over time if they are not engineered and built properly! Stay tuned!😉
> 
> PS, stop arguing about it! Many of us have tried alternatives and all samples have failed miserably, PERIOD! PS, you are arguing with some actual professionals in the industry also so perhaps you should take their advise and pull your head from your dark spot for just a second! And surfboards ain’t skiffs and while they take a pounding, the forces generated are not comparable to what a skiff sees! Apples/oranges my friend! Carry on and build it if you want, I’d like to see it!


JC, 
Don't waste the money. The guy has not even generated a single question based on the info given to him. He has only continued on with his schilling.

This thread will die and he will never build a boat made from insulation. He won't like our answer's and will start trolling other sites until 2022 and his progress on his build will be exactly where it is right now. NOWHERE!

I'm done. You and FC should just check out too. Complete waste of time and energy.


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## SurfDuffer (Dec 23, 2020)

JC Designs said:


> Ok, I’ve had enough! I will personally grab a sheet of 3/4XPS foam and do a side x side using carbon core 5lb pe foam “most economical core foam available” and do some flex/peel/MOE/and the good old fashioned BFH tests provided the XPS makes it past the flex test! Will laminate both sides with [email protected] 0/90 & 1 @1708 45/45 resin will be epoxy! Maybe I too can save a small fortune on my personal stuff since I would never dream of endangering a client’s life or his/her loved ones using F’ing insulation for a structural core in a vessel that takes much more abuse than the so called battle wagons! Wash board ripples create a harmonic effect that will straight up destroy boats over time if they are not engineered and built properly! Stay tuned!😉
> 
> PS, stop arguing about it! Many of us have tried alternatives and all samples have failed miserably, PERIOD! PS, you are arguing with some actual professionals in the industry also so perhaps you should take their advise and pull your head from your dark spot for just a second! And surfboards ain’t skiffs and while they take a pounding, the forces generated are not comparable to what a skiff sees! Apples/oranges my friend! Carry on and build it if you want, I’d like to see it!


Do me a favor and score both faces of the XPS foam with a wallpaper roller. Do a stress test to see what it breaks at when fully cured. Cheers.


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## SurfDuffer (Dec 23, 2020)

DuckNut said:


> JC,
> Don't waste the money. The guy has not even generated a single question based on the info given to him. He has only continued on with his schilling.
> 
> This thread will die and he will never build a boat made from insulation. He won't like our answer's and will start trolling other sites until 2022 and his progress on his build will be exactly where it is right now. NOWHERE!
> ...


Schilling? The funny thing is, my opening post simply asked if anyone here had attempted it. Rather than just say no or just ignoring the thread because you didn't have anything to add we had people chiming in with opinions and some of these people I'm not 100% convinced even know what XPS is then we had others who want to lecture or mock. I'm glad you're done. You didn't have anything to add to begin with. Cheers.


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## firecat1981 (Nov 27, 2007)

Yep think I'm done too. I waited for you to post up any actual data backing up your outlandish assumptions, but no go. Go for it sport, post up the whole process, including your tests. Proof is in the pudding. If you have the courage of conviction you'll start in short order so we can see.


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## JC Designs (Apr 5, 2020)

Peace, I’m out too! Best of luck to ya! Please share your work!


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## permitchaser (Aug 26, 2013)

Just saw 👀 a commercial on flex paste. Just spread out over some wire screen and you have a skiff suitable for a small tiller. Who knew


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## SurfDuffer (Dec 23, 2020)

Can't wait for those test panels.


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## texasag07 (Nov 11, 2014)

This thread is about as worthless as the guy wanting to build a diesel inboard on panga then saying he was going to change his plans to two inboards or something dumb for a project that was never going to happen. Surfduffer has provided no real world examples of this working besides surf boards. I can’t wait for the build thread to start.


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## Sublime (Oct 9, 2015)

At the end of the day, the Carbon Core foam in my build will be about *6%* of the total cost. Do you really want to toil for hundreds of hours and always have to worry about XPS holding up?


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## SurfDuffer (Dec 23, 2020)

Here is the original thread topic. Please read closely:

"Been looking at some of the foam core builds here and was wondering if anyone has done one using the blue, green or pink XPS foam you can buy at any home improvement store? It is completely compatible with Epoxy. There are some tricks to working with it but it would seem to me to be useful for one of these builds."

So to sum things up. Nobody here has tried it.


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## ShugC (Apr 23, 2016)

SurfDuffer said:


> Here is the original thread topic. Please read closely:
> 
> "Been looking at some of the foam core builds here and was wondering if anyone has done one using the blue, green or pink XPS foam you can buy at any home improvement store? It is completely compatible with Epoxy. There are some tricks to working with it but it would seem to me to be useful for one of these builds."
> 
> So to sum things up. Nobody here has tried it.


Are you intentionally being obtuse? you asked the question, then 3 peoples answers that it is not suitable for skiff building then you come back with this,



SurfDuffer said:


> Actually this is why I bring it up. With a few design accommodations the delam issue is not an issue. In surfboards the delam issue was mostly due to very thin skins and people leaving the surfboard in a blazing hot car and builders who did not provide vents in the boards, XPS is 100% compatible with epoxy. Structurally it is every bit as strong as some foams used in boat building applications. In fact it is more dense and harder to compress than some foams used in boat building. Obviously you're not going to use it for transoms but I could easily see it in coring hull sides, bulkheads, floors and such in micro-skiffs. Let's not forget that the foam is not where the strength resides in composite construction. The strength is in the skins. The foam is the web of the I-Beam.


You then get called on the FACT it is not anywhere even close to structurally sound to use as a boat core and you just keep spouting off. If your so sure it is safe to use then buck up buttercup and build a skiff out of it and prove the actual experts giving you advise wrong.


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## SurfDuffer (Dec 23, 2020)

If you use XPS you will die. LOL.


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## K3anderson (Jan 23, 2013)

Just wanted to revisit this. Did you end up doing anything? What were the results?


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## SurfDuffer (Dec 23, 2020)

K3anderson said:


> Just wanted to revisit this. Did you end up doing anything? What were the results?


I haven't but here is an XPS skiff by someone who did.


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## Surmonttfishing (10 mo ago)

SurfDuffer said:


> I haven't but here is an XPS skiff by someone who did.
> 
> View attachment 184741


Hello, that looks amazing. did you ever build yours? I've been toying with the idea...

don't understand how all the people say it can't be done when there are a ton of videos in youtube with people building skiffs and kayaks from xps.

truthfully I would like some more info on it as I have never built a boat or anything like it


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## SurfDuffer (Dec 23, 2020)

Surmonttfishing said:


> Hello, that looks amazing. did you ever build yours? I've been toying with the idea...
> 
> don't understand how all the people say it can't be done when there are a ton of videos in youtube with people building skiffs and kayaks from xps.
> 
> truthfully I would like some more info on it as I have never built a boat or anything like it


I haven't. My focus has been on surfboards and tinkering with my Mako. Recent build for my 16 year old on the rack and in action.


__
Sensitive content, not recommended for those under 18
Show Content











Laugh out loud. The forum software must think the surfboard I shaped is sexy since it made the picture sensitive content for people over 18 to view. 
























Here is a micro-skiff build by someone else that the guy just launched I just found with a quick YouTube search. He didn't run into people that told him it couldn't be done and it appears he didn't die for trying.....you with know all those crazy forces that a micro-skiff runs into on flat water compared to a surfboard in solid waves.

If you watch his glassing video you will notice that he is using the same techniques that I'll use when glassing a surfboard. He's just using two layers of 1708 instead of the layers of 4oz E cloth or S cloth I use on surfboards.


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## SurfDuffer (Dec 23, 2020)

The video series of this build is worth watching especially for those who claimed it can't be done.

*DIY MICRO SKIFF PART 2 sanding and shaping the xps foam*


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## bcblues (Apr 10, 2018)

SurfDuffer said:


> I haven't. My focus has been on surfboards and tinkering with my Mako. Recent build for my 16 year old on the rack and in action.
> 
> View attachment 200007
> 
> ...


That is a pretty sexy board. I had to laugh!


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