Will Gelcoat adhere to Epoxy?

[seafox]

It was a small repair on a trailer sailer and I used car bog!

[seafox]

Saw some boat builders using it and thought it would be ok. I got some shrinkage after I had painted it and got a hairline crack around the extremeties of the repair.

[Alan Wheeler]

Yep that is what you use and it shouldn't have shrunk. It must have been some cheap stuff perhaps. However, if the surface was subject to flex, it may crack. There are Bogs available that have glass fibre in them which helps with strength and flex and preperation of the edge with styrene liquid is recomended. It disolves the surface of the old Polyester and results in a better bond.

[seafox]

I went to a Resenes boat painting evening a few months back at Lowry Bay Yacht Club and the technical person there (from Resenese) told everyone to use expoxy for repairs and mentioned that the bog shrinks. Said that boat builders use it because it drys quickly compared to epoxy and you can paint over it quickly. Reckons it fails all the time.

[Brent Swain]

Epoxy sticks well to gel coat. gelcoat or any polyester doesn't stick to epoxy. About the only thing that sticks to epoxy is epoxy, wet on wet, within 48 hours of the last coat, or if the epoxy drys too long , then it has to be sanded between coats.
Brent

[GordMay]

Brent:
You seem to have fallen prey to the common misconception that gelcoat will not bond to epoxy. - gelcoat can bond to epoxy

I suggest you check the literature, there’s more documentation than the (2) WEST System articles, cited in the original post.
I’m particularly distressed when, otherwise knowledgeable people of reputation (such as you), post un-substantiated and inaccurate opinions (as fact).

[cat man do]

I would have thought that polyester gelcoat would stick to epoxy, like polyester resin sticks to timber.

With limited success.

Dave

[seafox]

From the www.epoxyglue.com site: "You cannot use auto filler (polyester bog) on your boat. You may experience adhesion problems on a flexing hull. You will have little protection if water gets to it. Heard of osmosis? There could be shrinkage problems. There could be shadowing with un-reacted styrene monomer, a solvent in polyester bog, reacting with the top coat. EPOXYBOG, our name for epoxy fairing compound, eliminates these problems. The secret is in using epoxy resin as the base. Epoxy reacts with, in this case, an amine hardener, to form a densely cross-linked polymer with exceptional adhesion and strength. It is a pre-filled, trowelable, epoxy putty, that feathers out and sands easily. It is very strong because it chemically cross-links to cure. Adhesion to a properly prepared substrate is exceptional.
EPOXYBOG, an Xtra lite, and creamy low density trade epoxy."