# Need help determining about fillers using cabosil or milfibers



## mudd_minnow (Oct 13, 2010)

Hello, I need ideas about filling a gap between the cap and bottom hull of my skeeter boat. I want to fill the gap and then fiberglass over the section and sand it down. Here is a drawing I've made to show the gap. #1 (Black and red) is the hull and cap. #2 is the cap filler I want to add. The blue is going to be the matt cloth x2 and 1708, or how would you do this?



My question is...Should I just use milfibers or add cabosil to the mix. I need to make sure that it has a lot of strength so if I do sand into the filler I would still be ok.

Thanks for any help.


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## firecat1981 (Nov 27, 2007)

I would use microfibers, don't liked milled, and wood flour or cabosil/fumed silica filler to plug the gap. Then before it fully sets up lay the glass, but 2 layers of mat should be more than enough, the 1708 seems like over kill to me. Then add some fairing compound and sand. If you are sanding into the glass you have gone too far, you should never go through the glass into the filler.

That's just my opinion though, I'm sure others will chime in.


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## Brett (Jul 16, 2008)

I've used phenolic microballoons on a similar project.
Not as heavy a filler material, in an attempt to cut weight.


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## DuckNut (Apr 3, 2009)

Muddy - Your post is contradictory - it contains products for epoxy and products for ester...

Cabosil will make sanding very difficult. Phenolic balloons are easy to sand and then there is a whole list of ones in between. Never liked wood flour.

Mat is no good with epoxy. 1708 has mat on the backside.

Milled fibers are best for ester resin. Chopped glass will make for a very hard product if allow them to dissolve first.

Which are you using then I can give you and educated opinion?

Also, why do you need have "to have a lot of strength"?


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## firecat1981 (Nov 27, 2007)

Edit to my previous post, For some reason when you said milled fibers I was thinking chopped, milled and micro are the same thing.


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## mudd_minnow (Oct 13, 2010)

> Muddy - Your post is contradictory - it contains products for epoxy and products for ester...
> 
> Cabosil will make sanding very difficult.  Phenolic balloons are easy to sand and then there is a whole list of ones in between.  Never liked wood flour.
> 
> ...



DuckNut,
The area I want to strengthen has a gap between the cap and the hull. I want to add a strong filler in the gap then fiberglass over the area to seal it. I'm looking for ideas of what to fill the gap with. I've seen on youtube about adding caposil to a epoxy resin to give it strength but understand it is very hard to sand. The fiberglass will cover the area and seal the hull from getting water. Brett mentioned that he used phenolic microballoons on a similar project. 

I did see on youtube the guy was using it on epoxy resin but my understanding is that it can also be used with poly resin. All my construction is using polyester resin not epoxy resin. 

I'm trying to choose between milfibers or microballoons to add to the polyester resin to give me a good strong filler and something that the matt will bond to. The filler needs to be the consistancy of toothpaste so it wont run. 

Thanks to all for input.


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## firecat1981 (Nov 27, 2007)

I'm not really understanding why you would need any real strength there either, but why not leave it and add a rub strip over the area to hide it since it's gonna get chipped up anyway on that style boat.

I've used milled fibers with polyester resins before, but mostly with epoxy and for strength it's my go to filler. It gives a lot of strength, but also does not make the mix brittle. It's not the smoothest, but if it's under glass then it doesn't matter. Still seems like a lot of work for little benefit.


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## trplsevenz (Oct 29, 2012)

For gap filling as you want, I would just add milled fibers for strength, glass microspheres for sandability, and cabosil to keep it from running out until it kicks. I dont think you should get really technical with the mixture, just do the first batch until you get the consistency that you like, but mix the first 2 ingredients befor the cabosil. Then a couple layers of 1.5oz mat should be enough to cover the cap/hull joint. if you have fairing compound, then just hit that over the mat after it starts to kick, and you wont need wax.


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## mudd_minnow (Oct 13, 2010)

Right at the cap-hull intersection is the water line. Also, this boat is very old and the flex of the hull - cap is (over the years) at this location. I wanted something that will add stength, water proof, and fill in the gaped area. 

The caposil adds to the thickness of the polyester resin/ milfiber (or microballoons) mixture so the mixture will not run while applying it. 

My next question is the mixture ratio. 
1 scoop of millfiber/microballons ( The size of the cap on a spray paint ) to how much polyester resin. I guess I'm looking for the consistancy of _?_. 

Now,
If I can get the consistancy of toothpaste, then I won't need to add any caposil to thicken. If I can't make that consistancey then add the caposil to make it a paste consistancy.

The length of area I'll be covering is around 30 ft long and 1/4 inch gap size.


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## Brett (Jul 16, 2008)

Using just microballoons as a filler, the ratio by volume is about 1 to 1.
That'll result in a mix like creamy peanut butter. Add a touch more of the 'balloons
and the mix gets stiffer, more like bondo, a dab more 'balloons and it'll stand on it's own.
Remember, the polyester resin has to be catalysed/mixed before adding the balloons.
Don't add all the 'balloons at once. It'll clump up. Better to add a spoonful, mix it in,
add a little more, mix some more, until desired consistency is attained.
Make a cup of mix at a time and use it immediately, so it doesn't set up in the container.


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## DuckNut (Apr 3, 2009)

Your answer makes it much clearer.

Two ways you can do it. Use the balloons/cabosil/fibers in any combination you want. There isn't a set ratio because all resins are different consistencies. So just mix it in the resin until it is toothpaste and then add the drops of MEK - those few drops won't affect consistency.

Other way would be to use a pre-mix marine filler. Already toothpaste (kinda idiot proof). Easy to work with and easy to sand. Most of the "bondo" type putties contain talc and talc will absorb water so they are not really intended for waterline use. But in your case the cheap stuff will work because you are going to be covering it. Just get a marine type.

Now the 1708 issue. way too heavy for what you need. Drop down to 1208 (0/90) and use a thin mat over to cover pattern. Use a couple layers if you are going to sand. It's ok to sand the mat to get a smooth finish but don't sand the biax. If you use 1708 you will be adding a gallon of resin. Also 0/90 will give you a real nice layment, 45/45 will be very rough when you are done. 45/45 is not a cloth you want to have in areas of sight as the pattern is rough and the stitching also shows.


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## mudd_minnow (Oct 13, 2010)

Thanks for the help. Is there a way to slow down the reaction time like using a slower bonding agent (Hardener)?


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## DuckNut (Apr 3, 2009)

Absolutely. Typically poly is mixed at 1 1/2%. In the heat you need to drop that down. Read the can and it will tell you how much to add to make it 1% or even .75%. Too much MEK in the heat can cook your project. Recipe is usually printed on the can of resin along with a min/max ratio.


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## mudd_minnow (Oct 13, 2010)

Thanks for all the help.


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