WD12 Finish question / miocroballoons

After my first layer of fiberglass/epoxy on the hull, assuming I want to use micro balloons to make the final fill coat, getting rid of the orange peel surface, do I brush on the expoxy/microballon coat with a paint brush?    If so, what is the proper proporation of the mix?  I ordered material from CLC. 

Is this the preferred approach over just 2, or more epoxy coats?  If I understand other posts - microballons as an additive is a lighter and more easily sanded material / finish to use for the second coat.  Post sanding, I plan to use 1 primer coat and probably 2 coats of black paint on the hull portion. 

Any advice is appreciated 


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RE: WD12 Finish question / miocroballoons

there are a lot of approaches...so i will share how i have used microballoons in the past.

first, just to clarify what you seem to already know, is that you don't use microballoons if you are going to varnish....so it is used only when you expect to paint over it.

i use microballoons in lieu of a primer coat. 

so i would basically complete your fill coats (two fill coats after the initial glassing). 

then i would sand as smooth as i reasonably can get the surface.  i would then use a micro-balloon paste (about the thickness of mayonaise) applied with a plastic expoxee spreader (https://www.clcboats.com/shop/products/boat-building-supplies-epoxy-fiberglass-plywood/marine-epoxy-fiberglass/plastic-epoxy-spreader.html)  to spread a paste of microballoons across the surface (you should see very little of it....but it will get into the divots/scapes/pinholes, etc).  and then do a finish sanding after that.

after that, you should be ready to paint.  the main advantage, is that this is a quick way to get the final dents and divots out of the surface and it is relatively easy to sand.  it has the advantage over a primer in that it is completely water-proof and much sturdier than primer.

good luck....sounds like you are getting close.




RE: WD12 Finish question / miocroballoons

I use microballoons to fill the weave and fair anywhere that's being painted. It's easier to apply, easier to sand and cheaper than straight epoxy.

I usually use 2 coats - the first is thin, the second about like cold cake frosting. Application is with a squeegee.

Have fun,



RE: WD12 Finish question / miocroballoons

   Its been interesting to see the advice on using a micro balloon mix as a filler.

I have just tried using micro balloons on a centreboard in the hope of getting a really smooth fair finish.  I first wetted out the glass cloth with epoxy in the normal way,  Then once cured to the tack stage filled the weave with two coats of an epoxy micro balloon mix (runny slurry consistency).  I waited for it to cure to a tacky finish between each coat to get a chemical rather than mechanical bond.  The third (final coat) did go on quite thick.  A day or so later when I came to sand it fair the top coat flaked away from the middle layer.  There was no appreciable adhesion anywhere.

Is this because coats of micro balloon epoxy mix don't form a chemical bond or is it more likely to be one of the other reasons: e.g. amine blush that I did not spot.




RE: WD12 Finish question / miocroballoons


Epoxy/MB mix will form a chemical bond as long as it has enough epoxy in the mixture. If the mix is too dry it will be epoxy-starved and the layers won't bond.

The other possibility is that the mix had just barely enough epoxy but you didn't push hard enough to get the thick mixture to make good contact with the existing surface.

While surface contamination is a possibilty. the fact that your glass and 2 fill coats worked tell me that you have mastered surface prep. The new thing here was the thick coat of epoxy/MB so that's where I'd look for problems

If it was me. I'd sand off the failed layer and try again. This time I'd add a bit more epoxy to the mix - not much, just enough to get a smooth texture that goes on like frosting. I'd also make sure to push hard with the squeegee, or whatever applicator you're using. to make sure that there's good contact between the old and new layers. You can test by pressing the squeegee over a layer a couple of minutes after applying it. If the layer curls or comes up it's too dry. If it just flattens it's OK.

Good luck,




RE: WD12 Finish question / miocroballoons

   Thanks Laszlo

The dodgy layer has been scraped and sanded off so I have got a good key to try again.

I am a little puzzled by reference to the consistency of frosting (icing on this side of the pond).

How does that compare to the descriptions in the manuals: gravy, peanut butter etc.  Would it stand up in peaks or would it spread out under its own weight?



RE: WD12 Finish question / miocroballoons

Try Betty Crocker Icing that comes in tubs at UK grocery stores. Its consistency at room temperature is what I aim for when I make epoxy/microballoon mix.

When applied to the boat, the mix ends up looking like the cake above.It will hold a peak, albeit a softly rounded one, even on a vertical surface. Note that that's a very thick layer but I find it easier to apply a thick layer and then sand it smooth, rather than trying to apply a consistent thin layer.

Now I'm hungry.


RE: WD12 Finish question / miocroballoons

   Thanks fo the advice Laszlo.  

Success second time around.

To continue the baking theme:The filler ended up somewhere between a slightly runny frosting or a stiff ganache,  But was OK for a flat surface.  It went on nicely with a serrated spreader to control the thickness.  The coating flowed ran out slighly to a reasonably even layer. 


The ridges and furrows showed up during sanding 

But it did not take much effort remove them and to end up with a smooth fair finish.

RE: WD12 Finish question / miocroballoons

Looks good, I think you've got it!


RE: WD12 Finish question / miocroballoons

Slightly different question about microballoons.  I'm thinking of using a microballoon/epoxy mix on the inside of my cockpit,  I get that it won't be beautiful, but frankly, the inside of the cockpit is never going to be pretty given that this was my first time working with fiberglass and fillets. 

My goal is mostly to make sure that the boat is fully waterproof, and that there aren't rough edges to catch me. Using microballoons seems like it would save both weight and effort.

 If I did that, would I want to paint the inside of the cockpit?  Varnish the exposed section for UV protection if paddle without a skirt? Just leave it as is?  

RE: WD12 Finish question / miocroballoons

Hi Elb, 

if there is any concern about water-proofness on the inside of the boat, you want to paint the area with epoxy.   the microballoon slurry is really a fairing compound....not a water-proofing compound.

i also don't think that microballooning the interior is worth it.  issues like rough edges are not really handled by fairing compounds....if you try to approach it that way you may be making you boat very very heavy becuase of how much epoxy/microballoon mix you would be adding.

for what you describe, and the approach i routinely take, i would knock down any rough edges with 60 or 80 grit sandpaper on a sanding block....and ensure your fingers are safe and well above the sanding surface.  for this kind of operation i will typicall use a sanding block that is a rectange - 4 inches long, two inches high and one inch wide....the sandpaper is held tight on the four by one inch surface by folding it up the side so your fingers are up a full inch away from the sanding surface and safe from getting sliced open by any fibreglass shards.

with this sanding block you can very quick knock down any rough edges.

once the rough edges are knocked down you can paint epoxy onto those sanded surfaces to ensure they are still water-proof.   

at this point, you can decide if you want to paint or varnish.  i typically avoid paint on the interior surface as it destroys your ability to see if you have a problem.  if you has a small leak on a painted surface, you won't notice it until real damage is done.  you can detect wetness much faster and easier on a clear surface and part of my routine boat maintenance is taking a look on the inside to make sure all the wood looks good and everything is dry.

you do want to protect any exposed epoxy from the sun, so i will typically put some varnish in the cockpit area on any exposed surface (floors and hull sides) into the cockpit area in front of the seat to about six inches in front of the coaming. 

microballoons themselves does not create UV protection for the epoxy it is encapsulated in.....so microballoons does not solve the UV problem.

anyway, i am sure others will share their perspectives as well....but these are the thoughts i would share with you on your question.



RE: WD12 Finish question / miocroballoons

The only thing I have to add to what Howard said is that one of these rubber sanding blocks is an excellent investment, two or more are even better. At $6.00 they're easily affordable, hold the paper securely and keep your fingers out of harm's way. They're waterproof, nearly indestructible and last forever. The tapered design allows fitting the tips into tight spaces. With more than 1 you can keep them loaded with different grades of paper and easily switch between them.


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