The Rot Doctor


Subject: CPES question (over fresh fiberglass)
Date: Sun, 31 Jul 2005

Hi doc,

I've used your CPES product for some time and love it. I have a question, though, that I can't find answered anywhere in the Q&A section of the website (which is not to say the answer isn't there . . . there are so many questions and answers that I may just have missed it).

Anyway, here goes. I have used your product in the past in constructing wood/fiberglass kayaks. I'm now making my first cedar strip canoe. I used CPES as a "saturation" coat before fiberglassing the hull. It worked great. Now I have the fiberglass/epoxy on and I'm just about done sanding it for final varnishing. I have a few spots in the fiberglass where I had some wrinkles/bubbles, and to sand everything smooth, I had to cut a bit into the fiber of the glass. Not all the way through, so I don't feel a need to patch it. I'm thinking that the glass fibers now are not as protected from water penetration and I was thinking of putting on another coat of CPES on the exterior of the hull. That would mean over the fiberglass/epoxy, to seal off everything. Then varnish on top of the CPES. Any reason not to do that? (The underlying epoxy, by the way, is MAS).

If it is a good idea, does the MAS epoxy need to cure out more? They recommend 2 to 3 weeks before varnishing. It's plenty hard for sanding now (it has cured about 8 days). But I would not want to put varnish on it for another week or two. Should I wait that long to put the CPES on, or can it go on now?

Thanks much. It is a great product!
Virginia L.
Salem, OR


I would suggest that over the exposed glass fibers you apply at least three dab-coatings of the CPES to that particular area, allowing at least a day between applications. This will saturate the fibers and seal them off from the possibility of water absorption. This is a procedure that you can do whenever you wish.

I would wait the recommended three weeks before proceeding further. Then I would suggest the following steps: 1) Wipe the exterior down with a rag dampened in lacquer thinner; 2) coat the hull with CPES; 3) as soon as the CPES reaches the tacky phase (or shortly thereafter), apply your first coat of UNCUT varnish. The CPES will bond to the base epoxy and during the curing process will "lock-in" the initial coat of varnish. You can then apply subsequent coats of varnish at your leisure.

And feel free to come back if you have any additional questions.