[Rhodes22-list] Polyester Gel Coat Over Epoxy?

[Roger Pihlaja]

Ed,

I went onto the Evercoat web site & took a look at the MDS sheet and other
technical data for the product you mentioned.  Based upon the chemistry of
the Evercoat product, I think it will work just fine over the top of your
epoxy repair if you take the following precautionary surface preparation
steps:

1. The reason for the horror stories regarding the incompatibility of epoxy
and polyester is that unreacted epoxy monomers react faster with the free
radicals in the curing polyester resin than the polyester monomers react
with those same free radicals.  So, if there are any unreacted epoxy
monomers remaining on the surface of the cured epoxy; then, they will "kill"
the polymerization reaction in the polyester resin before it even gets
going.  Therefore, to assure all the epoxy monomers are gone from the
surface of the cured epoxy, warm the surface to about 120 deg F for 15-20
minutes with something like a hair blow dryer.

2. Previous posts have the discussed the formation of a waxy layer of "amine
blush" on the surface of the cured epoxy.  This waxy layer will prevent good
bonding between the polyester and the epoxy.  Therefore, it must be
thoroughly washed away by water wet sanding the surface followed by a final
acetone rinse.  The wet sanding will leave the epoxy with a nicely scratched
surface to provide "good tooth" for a solid mechanical bond between the two
materials.

If you follow these precautions along with the application instructions that
will come with the product, I think the polyester gel coat will work just
fine over your epoxy repair.

Good luck, Ed!

Roger Pihlaja
S/V Dynamic Equilibrium


----- Original Message -----
From: "ed kroposki" <ekroposki at charter.net>
To: "'The Rhodes 22 mail list'" <rhodes22-list at rhodes22.org>
Sent: Tuesday, January 18, 2005 8:34 AM
Subject: [Rhodes22-list] Roger, a boating question


> Roger,
> Evercoat makes a product, One Step Finish Premium Gel-Kote #5670, to
> repair gel coat.  It is an isophthalic polyester product. Its instructions
> state not to be applied over epoxy.
> If epoxy paste was used to fill and fair the defects, what coating
> needs to go between the filler and the finish?
> Specifically epoxy was used to seal the gouges and Marine Tex used
> to fill and level.
> What do you think a minimum temperature such a product could be
> applied in, curing time not a major factor.
>
> Ed K
> Greenville, SC, USA
> Addendum:  "Before you join a new venture, be sure to consider certain
> questions:  Is the new product or service really worthy?"  Marshall Loeb
as
> quoted by William Safire
>
>
>
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