dark area on plywood (pics)

[cirrus]

Hello,
I really learned a lot in this forum, know I would like to post a question for the first
time. Our boat is a 25 feet plywood boat build in 1965, we (3 owners) received it for
free 3,5 years ago.

http://segelboot.piranho.com/141-4169_IMG.JPG

The donor was the father of the former girl friend of one of my friends, he received
the boat late at night on the wedding party of this former girl friend (she married
someone else). This situation should have made us cautions, but it has not. Today I
know that boats are never for free, but at this time we thought that a little painting
would made us to boat owners. However, after two years of renovation, we realized
that things are not that simple. But we are sailing now for two summers, this
compensates a lot.

In brief, we stripped the fiberglass from the hull, because it delaminated in some
areas below the waterline. The hull was sound (bruinzel mahagoni plywood), we re-
fiberglassed the hull. Due to rot, we had to replace a small area of the keel where the
rudder comes out. Part of the transom was rotten due to 35 years of drilling holes for
fixing the outboard, we cut out this area, replaced it and added one piece of plywood
to the inside and one the outside of the transom for stabilisation. Interestingly, the
cockpit house and the cockpit are made from one piece large of fiberglass. This
fiberglass was weak, so we stripped the paint and the gelcoat and reglassed this
area as well. In the cockpit, we laid some teak (solid teak that was already glued to
ply)

http://segelboot.piranho.com/142-4216_IMG.JPG

Here begins the question: The deck was originally10 mm mahagoni ply. It was
painted and had 1 million drilling holes and some rotten areas. We replaced the
rotten parts and glued a new layer of 6 mm mahagony marine ply (ply with 5 layers)
with sikaflex onto the deck. This ply was stained and varnished . The first two layers
were 2 part varnish, then we switched to one part varnish (3 layer). After the first year
in the water, the deck developed some dark areas.

http://www.rolf-edv.de/boat/142-4215_IMG.JPG

http://www.rolf-edv.de/boat/141-4166_IMG.JPG

http://www.rolf-edv.de/boat/141-4165_IMG.jpg

The reason for this problem will be (as I learned here in the forum) that the end grain
was not properly sealed with epoxy or varnish. In some small areas, the varnish
became opaque, I my opinion it does not stick to the wood any longer. However, the
varnish doesn't peel off.

Now the question: If we are going to repairs this, can we sand the ply and re stain
and re varnish? Or will we sand trough the first layer of the ply? (5 layers, 6 mm ply).
And, if one can sand it, do we have to sand the hole sheet or can we just repair the
dark areas?

[Ian McColgin]

You correctly identify the cause and then discuss cosmetic solutions that don't address the cause.

First, if the ply is not rotten yet, get it dried out, maybe lift some of the covering and such. Strip all the finish off the decks. If you can seal those edges in situ, you'll get out of this easy. Use CPES and prepare to spend some $$ on quite a bit of it.

Call Mr. Smith to be sure about how to use stain with CPES so you get the whole shebang well sealed before varnishing.

Consider an alternative: varnish is a poor finish for that application anyway and the nice teak and such will be more nicely set off if you paint the deck bristol beige and have a good non-skid in the paint. If you're gonna do that, you could then go from CPES on the endgrain to a complete seal covering with a gooey stuff like Gluvit, let is cure a week or so, and you're set to paint.

G'luck

[WFK]

Trying to sand out an area and re-varnish in hopes of trying to make it blend is tough and with plywood, because of the thin laminations, near impossible. The problem lies beyond the stain, and the dark spots are signs of moisture intrusion and until you find the culprit it'll be a never ending battle. Looking at the picture where the back stay comes down, it's obvious that water is making it's way around the chain plate. In this particular case this piece of hardware needs to be re-bedded to insure no more leaking. As far as your plywood deck goes, after all these problem areas are taken care of, I'd paint it.
Good luck! Bill