The Rot Doctor


Subject: Caulking fir deck planks
Date: Mon, 20 Jan 2003


Love your column! Thanks for making this really good info available on the internet.

Need some advice. I have a 27' Maine-built all wood lobster boat (1949). I am installing 1" x 4" fir planking on the cockpit deck/floor in the tradition of Maine lobster boats. I need someone to recommend a primer/sealer and/or a caulk for between the planks; I want to make them watertight. I anticipate needing to seal the butt edges of the planks so whatever caulk I use will get a good bond. The caulk will need to be flexible and expandable, permanent and UV resistant. I know what you will recommend for the sealer, but what about the caulk? I only want to do this job once! Also, what can I put on the fir to keep the original wood look. I prefer not to paint it.

Thanks for your valued advice.
Paul V.

Well for caulk on fir I would recommend a good one part polyurethane. Something like our 3M™ 4200 which would work fine for your application.

As far as leaving the fir natural, I wouldn't recommend it. Sunlight will turn the wood gray, forcing you to sand the wood periodically. Water will be absorbed into the wood, causing the wood to swell and shrink considerably. This can cause two problems. First, the seams will be more likely to open up, requiring you to re-caulk. Second, splits will develop in the wood, allowing more moisture to get into the wood, causing more problems. Teak is one of the few woods that can be used to make a deck out of and that can be left unfinished. You are still dealing with the UV graying, and the wood can still swell and shrink somewhat, causing seam problems. There is no really good answer, but I would definitely be sealing the surface if I was using fir. Personally, I would use CPES as a sealer, and Elasta-Tuff™ 6000-AL-HS as a topcoat.