Help with fairing

[Emerger]

Hi all. I completed a transom rebuild and am doing the finish work on it including fairing. I mixed cabosil with gelcoat to thicken it to use as a fairing compound. It went on smooth and looks good but even after allowing it to cure for several days it seems to gum up my random orbital sander when I try to sand it. It seems to sand okay at first but then begins to gum up as I go along. I mixed the resin and hardener correctly but it just seems to be a little stubborn.

Any advice? Thanks

 

[saildan]

Re: Help with fairing

I mixed cabosil with gelcoat to thicken it to use as a fairing compound. It went on smooth and looks good but even after allowing it to cure for several days it seems to gum up my random orbital sander when I try to sand it.

Couple of things -

The kick and cure of pigmented polyester resin (gelcoat) depends some on temperature and a little on humidity so the amount of catalyst needed can vary daily. Some of nailing that down can be derived by mixing a small test sample before you mix the main batch and adjusting the ratio based on your sample findings. Some comes from experience - which you are gaining now.

If you under charged the mix for the ambient temperature during the cure duration, you might try heating the section with a heat gun for 20 minutes and checking it again a day later.

I don't quite understand the use of Cabosil for a fairing mix. Cabosil (fumed silica) is a hardener/thickener like rock aggregate is used in concrete. Normally a fairing mix uses softeners, like micro-balloons for easy smoothing.

Either way, if you are rubbing out to blend the repair into the surrounding gelcoat, fillers in gelcoat are going to make that spot always appear slightly dull compared to the adjacent surface.

It'll probably be OK for priming and painting over, but then why apply cosmetic gelcoat when you are covering it with cosmetic paint. In that case save money and just make your final fairing fill using regular resin.

.

 

[Sean-Nos]

Re: Help with fairing

Sometimes when the resin is curing you can get Amine Blush that can clog up the sand paper,try washing it with warm water first and then sand at a low speed or by hand.

 

[i386]

Re: Help with fairing

As previously stated. If it's gumming up your sandpaper, it's not fully cured. Resin thickened with Cabosil/Aerosil does not sand easily once it's cured. I've good results adding 3M glass bubbles to expoxy resin to make a sandable fairing compound. For a sandable filler, I add about a 50/50 mix of Cabosil/Aerosil and 3M glass bubbles to my epoxy resin. It's a little harder to sand but my thinking is it would be a little stronger.

 

[ondarvr]

Re: Help with fairing

Gel coat, like other polyesters needs to be sealed off from air to fully cure on the surface and not gum the paper. Also when its applied thin there may not be enough heat and energy generated to get a good cure.

If its just the surface that's gummy, you can wipe it with acetone, this should remove any uncured product.