The Rot Doctor


Subject: Waterproofing a sink
Date: Mon, 15 Feb 1999

Dr. Rot,

I recently built my daughter a miniature play kitchen countertop and sink (with running water) out of regular white pine. My plan was then to waterproof the wood with a two-part Marine Epoxy Resin (made by Bondo). I am concerned the resin may crack do to the expansion/contraction of the wood vs. the epoxy. I need the sink to be somewhat UV resistant since the sink will be outdoors and may be in the sun.

I am also confused by the differences/advantages between Epoxy, polyurethane, and spar urethane. Would one of these be better?

What do you recommend?

Mark in Texas


Your concerns are well-founded. The chances are very good that cracking and delamination of the epoxy resin would occur for exactly the reasons you suggested. This sort of thing is done in the Marine industry ONLY with the addition of fiberglass cloth under the resin and on premium cured wood, and even then it is not always successful. And of course it makes it a bigger project.

My suggestion would be to coat the entire piece with a penetrating epoxy, such as our CPES, which will give the wood some protection and yet still allow the wood to breathe, expand and contract without serious damage. I would do one coat on the inside and 2 coats on the outside. Following that, I would apply several coats of a polyurethane finish with UV inhibitors. Natural spar varnish would work as well, but you'd get a better bond with a polyurethane. Or you could use any paint over the CPES. The CPES acts as a prime coat for any of these finishes.

The epoxies and the urethanes are all polymer compounds. They're quite different, actually, but will bond together. The epoxies are by far the more expensive to manufacture, but are generally stronger and more enduring. UV inhibitors cannot be added easily to epoxy compounds, which is why the varnishes and urethanes are used for final coatings.

A CPES/polyurethane final finish should be fairly sturdy, have a clear varnish-like appearance and be repairable with a bit of sandpaper and a new coat of polyurethane. It will be UV-proof, assuming you use the right urethane. The can will say that there are UV inhibitors.

You can figure about 200 sq ft. per gallon for the CPES, so depending on the size you may need as little as our 2-pint unit. We ship every day via UPS GroundTrac, and you can order by phone, fax or on-site order form. Credit cards are accepted.

Come back if you have additional questions.