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Black Stains Under Varnish On 4yr Old Bay Window Plywood Veneer Window Seat Area

Black Stains Under Varnish On 4yr Old Bay Window Plywood Veneer Window Seat Area

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  #1  
Old 04-20-14, 08:39 AM
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Black Stains Under Varnish On 4yr Old Bay Window Plywood Veneer Window Seat Area

I think the Pratt & Lambert # 38 gloss varnish failed allowing moisture to penetrate the veneer and react with the tannin's in the veneer.

I have sanded the failed varnish area (3'x 5")down to the veneer and used Oxalic acid in 4 applications with mixed results. The black marks remain and are only slightly lighter. I have also tried household bleach which has not helped.

I'm aware of a 2 part wood bleach as a last option to remove the black, but I think it wood bleach the wood to much making a stain match impossible.

Another problem, the area where varnish was sanded/removed will not take stain. So can you guys please let me know your thoughts on the matter. Thanks.
 
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Old 04-20-14, 01:52 PM
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Often sanding alone won't remove all the existing finish and if all the finish isn't removed, it's hard for the wood to take stain. Generally when water stains won't sand or bleach out the only choices left are to either disguise it or replace the wood.

If you'd post a pic or two we'd have a better idea of what you are dealing with.
 
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Old 04-22-14, 08:38 AM
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Here is a link to a photo so you can get an idea of what has happened.

2014-04-07_1315 - Docfletcher's library

I hit the whole window seat area with 150 grit sandpaper, then used 000 steel wool to remove all vestiges of the finish. Then applied Oxilic acid in a slushy slurry and scrubbed the marks with a tooth brush and let it stand overnight. Mark's are almost gone with just a few very light remnants
remaining. Waited 24 hrs then applied the stain. I was pleased to see after wiping off the excess stain the surface accepted stain in a nice even fashion without blotch's.

After 6 hrs the 2nd stain was applied. When dried I noticed the surface has a light sheen to it. But it is not tacky or sticky so it might be OK. What are your thoughts? I'll get a photo up so you can see the final job.

Screen Shot Link Added...

2014-04-22_1055 - Docfletcher's library
 

Last edited by docfletch; 04-22-14 at 08:57 AM. Reason: Sheen Photo added
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Old 04-22-14, 10:20 AM
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Looks good. I'm not a fan of applying multiple coats of stain because stain is formulated to dry more by absorption than on top of any sealed surface, that makes it harder for subsequent coats to thoroughly dry. You didn't mention it but stain needs a few coats of poly or varnish to protect it from wear.
 
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Old 04-22-14, 11:14 AM
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Well, I intend to use the Pratt & Lambert #38 Varnish again. I do have some concerns though since I had the failure as shown in the 1st photo. I don't understand how the water got through the varnish. If you look closely at the first photo there are no white milky spots, the type usually associated with water getting through.
 
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Old 04-22-14, 11:21 AM
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Window sills and seats tend to get more sunlight than the rest of the interior woodwork. The UV rays will break down the paint or varnish over time. It's not uncommon to see moisture damage on these surfaces. IMO it's a good idea to sand these areas lightly and apply a fresh coat of finish every few yrs.
I assume there is decent insulation under the seat.
 
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Old 04-22-14, 01:42 PM
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There is certainly truth in that . For the 1st two years we did not use a seat cushion. The problem seemed to appear some time after the cushion was added. The window is a composite, high energy saving .04 or .06 so should be well insulated from the maker.

I think the cushion trapped condensation and caused the issue. The black marks even migrated to the cushion underside.

The window seat area gets very cold in winter. It makes me wonder if I should use use a long oil spar urethane on the seat area because it is possible expansion and contraction may be causing the current varnish to fail???
 
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Old 04-22-14, 03:29 PM
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Spar urethane doesn't dry to as hard a film as regular interior poly so that means it isn't as resistant to scratches and wear. Since you seem to have narrowed the cause down to the cushion, I'd get in the habit of removing it occasionally so everything can air out and not trap any moisture long term.
 
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Old 04-24-14, 07:06 AM
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I will stick to the Pratt & Lambert # 38 Varnish. It seems to be the thing to do since the overhead and sides were done with it.
 
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