The Rot Doctor


Subject: Product Recommendation (exterior varnish)
Date: Fri, 16 May 2003

We are using a mahogany siding on the exterior of a house and would like to use a marine coating on it. Do you have any suggestions on a product. We would prefer a clear semi-gloss finish the requires little maintenance. When the coating has to be reapplied it would be ideal if minimal scraping was required.


Your only real choice here would be our CPES (Clear Penetrating Epoxy Sealer) which is then top-coated with a marine-grade varnish. The CPES will greatly aid in the bonding of the varnish to the wood, and the varnish will have the UV inhibitors in it to protect the epoxy and the wood itself from deterioration/discoloration. At least 3 coats of the varnish would be required, and even more if the wood is in a sun-exposed area. There are no other clear finishes that have significant amounts of the UV chemistry, and it's the UV additives that are essential to protect wood.

The CPES will also protect the wood at its base level, and if the first coat of uncut varnish is applied as soon as the CPES treated surface becomes "tacky", or shortly thereafter (within 12 hours of the CPES coating), the curing epoxy bonds the first coat of varnish into the epoxy. This will help prevent future blistering/peeling. CPES coverage on good would is about 250 sq. ft per gallon, so it is not a major expense considering the benefits.

As a longtime boat owner and varnisher, I can tell you that the wood looks beautiful finished in this manner, but it has the downside of maintenance. The more coats of varnish that are applied, the easier subsequent maintenance will be. The reason for this is that re-varnishing for protection and appearance (which you will have to do periodically, the timing depending on the exposure of the surface to sunlight) requires light sanding of the surface. When this is done there should be enough varnish on the wood so that the sanding DOES NOT reach the bare wood level. I won't go into the details of why this is not desirable, but take my word that it eventually degrades the appearance to the point where the whole surface has to be taken down to bare wood and the complete refinishing process re-done.

Any premium varnish purchased in a boat store will work fine. There are two basic types, petroleum-based and polyurethane-based. Either will work. The polyurethane varnishes are a little harder and "brighter", and a little more expensive as well. Either type will work well with the CPES prime coating.

Hope this is some help, and feel free to come back if you have additional questions.