The Rot Doctor


Subject: What do I do now? (deck seam repair)
Date: Wed, 20 Oct 2004


In July of 2003, I purchased caulk, Elastuff 120 and Rhino Top to cover the boat deck on our research vessel. I applied the coatings in November, 2003 and it worked well that winter. During the summer, the seams started to open in small cracks and after yesterdays storm, it was raining inside the boat. It has been suggested that I put down marine plywood, caulk the seams between sheets, and recoat the deck again. Any suggestions? I am going to try to seal the open seams and coat them with the Elastuff before applying another coat of Rhinotop before it rains again.
Thank you, Byron

Byron R.
Redwood City, CA


Wood is one of the more difficult building materials to maintain. Regular lumber swells and shrinks a considerable amount with the movement of moisture into and out of the wood. That is one of the appeals of plywood. Having the layers glued together makes plywood a very stable building material.

I suspect that the original coatings were applied when the wood was relatively moist, and therefore somewhat swollen. Since the moisture was prevented from entering the wood in the winter, when summer came the wood achieved a dryness that it hadn't gotten to on previous summers. Moisture moves out of the wood very gradually, taking months rather than days.

Since the wood has now achieved equilibrium at a lower moisture content, if you were to repair the cracked seams you would likely have no further cracking issues. I would recommend taking a utility knife and some sharp blades and cutting out a vee into the caulk seams where the cracks are. I would then prime the crack with CPES, and when this cures for 24 hours fill the vee with some polyurethane caulk like 3M™ 4200. This would certainly be easier than filling with Elastuff, and then could be top coated with Rhino Top as you suggest.

If I am right, and the wood has stabilized at a lower overall moisture content, this should be a lasting repair. If you have further trouble, you could certainly go to an overlayment of thinner plywood. The stability of plywood would reduce the chance of future problems, except for at the seams in the plywood.

Come back if you have further questions.