Mold! Defective Plywood?? - Woodworking Talk - Woodworkers Forum
 
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post #1 of 6 Old 04-12-2016, 10:29 AM Thread Starter
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Mold! Defective Plywood??

Hopefully I put this in the right place...

I have some plywood that is mysteriously molding. Is it defective? Why would it mold?

Over a year ago I build a small box that doubles as a table for the back porch. The porch is covered and not directly exposed to the elements. The 'box' part of it is for storage. It was painted in outdoor paint. The overall paint job is good as all the non-plywood parts are not molding.

I also used used the same can of paint to paint a hand railing for the front porch that is not covered and gets the full sun as well as all the rain. It is still fine, no mold, peeling, etc. So the paint is good.

As you can see in the first attachment, the plywood is very moldy on the back side. And the other plywood panels are molding, too, but not as bad.

So I looked at the 'scrap' leftover out in my very dry storage shed. It is the only thing out there that has any mold on it. This piece has never been subjected to the elements. This is the second attachment.

I've never seen plywood mold. All I can think of is it is somehow defective.

Any ideas, thoughts, or comments?
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post #2 of 6 Old 04-12-2016, 11:57 AM
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Your problem is the paint is exterior but the isn't. A certain amount of moisture will go through paint especially if it is latex and cause the wood to rot which is a breading ground for mold. Even exterior rated plywood is common wood veneers glued together with a waterproof glue. The wood veneer can as easily rot as common wood. Only pressure treated plywood can give you some protection however the right paint is more important. The best would be a marine grade epoxy coating. At the very least I would use a exterior oil based enamel and put a couple extra coats on.
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post #3 of 6 Old 04-12-2016, 09:50 PM
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Is this an isolated case?Meaning,is there,or has there been mold issues on your property or your your neighborhood now or in the recent past?
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post #4 of 6 Old 04-13-2016, 09:54 AM Thread Starter
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Quote:
Originally Posted by Steve Neul View Post
Your problem is the paint is exterior but the isn't. A certain amount of moisture will go through paint especially if it is latex and cause the wood to rot which is a breading ground for mold. Even exterior rated plywood is common wood veneers glued together with a waterproof glue. The wood veneer can as easily rot as common wood. Only pressure treated plywood can give you some protection however the right paint is more important. The best would be a marine grade epoxy coating. At the very least I would use a exterior oil based enamel and put a couple extra coats on.
Thanks for the reply Steve. I can see how a tougher coating would help, but why would the plywood in the outbuilding mold? See below for more on this.
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post #5 of 6 Old 04-13-2016, 10:02 AM Thread Starter
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Quote:
Originally Posted by BWSmith View Post
Is this an isolated case?Meaning,is there,or has there been mold issues on your property or your your neighborhood now or in the recent past?
Hi BWSmith. Yes, this is an isolated case. No, there are no mold issues on the property, or in the house, or in my other storage shed.

Also, no other plywood shows any sign of molding; only this piece. This plywood is relatively new, a year or so old (I'm bad on time, sorry). In my other storage shed I've had plywood out there for 10-15 years and nothing molds. Unlike the dry shed where the moldy plywood is in, this other shed had some moisture problems until I put a new roof on it. It has even 'flooded' out a bunch of times when it rains hard enough, getting 1/2" of water in it. Still, even when it was quite humid in there, and until it drys out from the minor flooding, nothing has ever molded.

Last edited by TwoRails; 04-13-2016 at 10:06 AM.
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post #6 of 6 Old 04-13-2016, 11:06 AM
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Originally Posted by TwoRails View Post
Thanks for the reply Steve. I can see how a tougher coating would help, but why would the plywood in the outbuilding mold? See below for more on this.
Mold is just an organism feeding on the wood. Any way moisture can get to the wood even from excessive humidity that is all it takes for mold to plant it's roots.
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