Re: Paint Chemistry

Posted by Terry Mcadams on May 28, 2004

Paint chemistry isn't my field (but I can tell you a good bit about pollution chemistry). Anyhow, I've encountered the problem that Sam Devlin notes with alkyd paint/varnish over epoxy a few times over the years. Refraining from using any organic solvents in favor of a water or vinegar/water wash down seems to help.

Note also that, if you paint late in the day, the ambient humidity may not be as critical as what's going on in the early morning when the cold hull contacts warming air. I suspect you might get a very moist micro-climate just at the hull surface - not the best thing for any finish. Thinning helps abate this hazard to your finish.

The comment on old paint or even a bad can makes sense. I suppose old finishes can undergo a critical chemical change in the can, but the gradual loss of solvent may be the more common cause. I always thin at around 10% with recomennded thinner for any finish - more if the paint still feels stiff or doesn't seem to want to flow out nicely. If it's damp and/or I'm doing vertical surfaces, I use 5% lacquer thinner to speed drying and avoid runs and sags.

I always put a freshly painted boat out in the sun for a few days to a week or more to cure. Seems to be the best way short of an industrial oven. If the weather is bad, I try to heat the hull from the inside, on the (unproven, by me anyway) theory that any finish needs to dry uniformly from the undersurface out to achieve a timely full cure.

There is a very good article in Wooden Boat this month about boat painting. Lots of practical advice from a very wise and patient professional painter.

Gees, if we keep searching for the perfect cure, we may all win the Nobel prize someday.


In Response to: Re: I'm Watching Epoxy by Steve Miller on May 27, 2004