Resealing a marine plywood deck

I sail a flying dutcman dinghy that has a varnished plywood deck. I have recently moved to the coast and the salt water has caused the deck to deteriorate with black spots appearing in the wood. I will have to sand the deck down and reseal and would appreciate any advice on the products and process that would ensure a long term solution.

 

The standard way of doing this would be to varnish with a standard oil-based varnish. Alternately, you could apply a two-part polyurethane clear finish. The two-part is much harder and it lasts longer while regular varnish is easier to apply and touch up. Both have UV inhibitors to resist deterioration from the sun.
I personally like the simple old style "spar varnish", or one-part varnish. If you're careful, touching up religiously whenever there's finish scraped or worn off (and it helps if the boat is covered when not in use), and you recoat every season, the finish can last for a very long time----- potentially ten years and more.
If you're not a maintainance type guy, the two part might be a better choice. Like a car's finish, it looks great when new but after a few years is begins to show its age. Single-part varnish looks brand new every season.

 

The black spots are wet wood or possibly more. To further what Alan said, ask yourself if you really want this type of finish, as it requires considerable upkeep. A painted deck will let you have fun a lot longer with a lot less worry. No, it doesn't look as good, but it can also have texture added, to make under foot traction more predictable.

 

You probably won't be able to get rid of the black spots by sanding. Maybe some chemical bleaching agent will help but it's hard to get an even color or usually do much more than lighten up the black spots to any more than just a generally dark color.

 

the spar varnish will last longer if applied over a thin coat of epoxy (thinned or un-thinned)

a win-win combination - the epoxy provides a stable surface for the varnish and the varnish provides UV protection for the epoxy.

have also seen professionals really thin the spar varnish for the first few coats (better penetration) then unthinned spar varnish for the last few coats. this is instead of the epoxy basecoat.

paul

 

Using epoxy under varnish has to be done with the understanding that any exposure of the epoxy to the sun due to inattention such as not religiously recoating the varnish when needed could lead to a chalky fading (destruction) of the epoxy, which can't take direct sunlight. In addition, coating the exterior with epoxy without coating the interior creates a situation where moisture can enter from behind and become trapped under the epoxy on the outside.
Therefore I'd recommend just going with varnish unless you plan to coat the interior as well.
Absolutely thin the varnish first as mentioned. Then expect to lay on at least a half dozen coats at a minimum with an additional two coats each season to maintain it. Or use a two-part varnish and get a few years of zero maintainance followed by a repeat of the job to renew.

 

I agree with Alan, in that a straight clear coat (alkyd or polyurethane) are preferable to an epoxy under coat. There's no real adhesion advantage that can justify the cost, effort or repair possibilities.



Get this book if your serious about your varnish job. A long term solution isn't possible with clear coats on wood. It's just the nature of the beast. I own a 24 year old runabout I built in the late 80's that's only had two varnish jobs on her. The only way I can get away with this, is to store her indoors and keep her clean and dry. If the boat sees much use and/or is stored outside where direct (or indirect) sun light can hit it, you're in the same boat as everyone else and recoating/touching up annually.

 

I typically start with a couple of coats of unthinned epoxy on raw wood mainly because i can block it down and have the grain filled to glass smooth with a lot less coats than using all varnish, then i will use at least 3 coats of spar varnish, usually Flagship. As has been pointed out you MUST do any repairs to the coating asap but this is so even if using all varnish. To me clear coatings should be reserved for things that are easy to sand and refinish otherwise they tend to get neglected, the large deck area of your FD is easier to deal with than silly little pieces if trim seen on some cruisers. I would spring for a fitted boat cover to keep the uv off for the 99% of the time when the boat is not being used unless you can keep it indoors. Over time it will pay for itself .

Steve.

 

It is all how you tend to use and maintain the boat.

I have had Flying Juniors, which usually suffered from the same problems when bought. After removing all varnish, I treated the wood with oxalic acid. First only the stains, when satisfied a treatment over the complete boat. After that, rinse with water. Then repair the areas that did not cope (rotten places, or places that have enthousiasticly been perforated with holes of all sizes, previous repairs not executed properly, damaged areas)
Small dings can be worked out by carefully removing the paint, then drip water in the dent. Keep doing that till the ding almost at surface level again. This will dramatically reduce the visibility of the ding.

After all woodwork is well, I applied epoxy in the following way:

-apply thin coat of epoxy (the foam rollers of West System help in achieving a nice thin coat)
-allow to cure
-sand (the fibers came up). At least 75% of the surface should be matt.

-apply epoxy. Do the obvious lows first (old dings) so they can fill somewhat.
-allow to cure somewhat
-apply epoxy (wet in wet, or should we say "wet in rubbery". Usually after an hour or 4-5 or so)
-allow to cure
-sand (85-90%)
-apply epoxy
-sand (100% matte)

Apply a varnish which has UV filters in them. (I usually spray painted 2 layers of 2-pack PU varnish)

Keep in mind that I first repaired all leak paths or suspicious leak paths, and that, after sailing, these boats were flushed and drained, and stored inside or under a well fitting tarp, inspection covers open, to allow some ventilation.
Usually the boats were so airtight, that after sailing, when opening a drain plug, it made a hissing sound, due to the pressure difference.

Insides of these wooden boats were treated with 2 layers of regular oil based varnish. (And as these inners were not reachable, I never worked on that)