To Coat or Not to Coat?

Posted: July 1, 2013

By: Frank Lanier

I have a Fisher Pontoon boat whose wooden deck’s topside is covered in vinyl. Would it help or hurt my deck’s longevity if I were to apply some type of wood-sealing product to the underside of it?
Not surprisingly, there are a lot of differing opinions in the “to coat or not to coat” debate. Those against it typically quote the same mantra: Wood that can breathe will expel moisture better and last longer. While that may be true in some cases, plywood is a wood product, but it doesn’t act like solid wood and shouldn’t be treated as such.

Unlike solid wood, plywood is dimensionally stable and doesn’t need to breathe. Even so, the wet/dry cycle can cause “checks,” which allow moisture, fungus and bacteria to penetrate the wood, leading to problems such as delamination and rot. As such, sealing it is a good idea.

Epoxy is probably the best choice for total waterproofing. Paints, preservatives and other similar coatings will offer varying degrees of protection as well, though they may have to be renewed occasionally. Coatings with a fungicide will help stabilize the moisture content in the wood and also prevent organisms from thriving.

My recommendation would be to add three or four coats of a quality epoxy, possibly followed by a compatible paint, if color is desired. The first coat or two of epoxy can be thinned (based on the manufacturer’s recommendations) to aid in penetration of the wood.

Total encapsulation of the plywood deck (i.e., top, bottom, sides, fastener holes) will work best, although that’s typically not an option unless it’s done before the carpet or vinyl coating on top has been installed (or is being replaced). Carpets and vinyl coverings can trap moisture between them and the plywood — a leading cause of rot and deck replacements. If you can completely encapsulate the deck prior to carpet or vinyl installation, your grandkids will probably be the next ones who have to deal with deck issues.

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