drywall mud and plywood

tbo123June 9, 2013

Sort of in a quandry.

I'm patching a plaster wall that is about 3/8" plaster and brown coat over 3/8" drywall. (so, 3/4"). I was going to use 1/2" drywall over 1/4" plywood to patch.

My quandry is that the edge of the plywood meets with the edge of the 3/8" drywall under the plaster, so that my 1/2" drywall is proud of the edge. It's flush to the wall and the edge of the 1/2"drywall would get plenty of mud but it would seem to mostly be joining with the brown coat and plaster and not the 3/8" drywall. Hope that makes sense.

I was thinking of stacking 1/2" and 1/4" drywall or 2 layers of 3/8" drywall but the plywood is just a tad oversized which works good for me.

Anyway, will the bond be as strong using the plywood?
(Using EZ Sand and paper tape)
Here's some pics of what I'm trying to say.

just holding a piece there for demonstration.
Thanks

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brickeyee

You want the base materials to be slight thinner tan the finished.surface.

I would just use 1/2 in drywall and EasySand to make up the1/4 inch.

This layers of regular premixed mud put a lot of water into the substrate, and then shrink horribly as they dry.

If you mix EasySand about as thick as peanut bitter it has very little shrinkage and you can fill that area in mostly one coat with a finish coat (or two) to smooth it off.

If you use Easysand20 you can easily get in two coats per day.

It does not have to be dry for the next coat, just set.

Put a blob aside and when that is set put the next coat on.

    Bookmark   June 9, 2013 at 12:23PM
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tbo123

Thanks Brickeye.

My question was regarding the integrity of the bond between the plywood and the mud.

I would have to do about 15" of 1/4" thick EZ sand your way. Seems a little much.

With the plywood it already is about an 1/8th shy from level at the seam.

    Bookmark   June 9, 2013 at 12:48PM
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live_wire_oak

Plywood, even though it's fairly stable, is still wood, and is still hygroscopic. It will absorb water and expand. Then shrink when it dries out. Not good in a patch for short term or long term.

I agree with Brickeye. Use the EZ Sand and drywall only. Maybe some fiberglass mesh to increase the strength.

    Bookmark   June 9, 2013 at 1:18PM
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brickeyee

Wood is NOT a GOOD base for mud or plaster.

Even the old wood lathe relied on the 'keys' of plaster forced between the gaps of the wood for most of the strength.

1/4 inch thick layer of EasySand n top of drywall is nothing.
I have used it over an inch thick to replace plaster grounds wen required.

If you want some extra assurance, cut Elmer's white glue 50% with water ad paint it on the drywall.

Wait for it to become tacky and then apply the first layer of Easysand.

This post was edited by brickeyee on Mon, Jun 10, 13 at 9:32

    Bookmark   June 9, 2013 at 4:49PM
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tbo123

Thanks
Ok, I don't know if this is bad etiquette or not but I can't get more than 1 pic posted here.
So, below is a link to another site with more pics of what I'm trying to explain:

I understand that wood is not a good substrate for plaster or mud. The plywood is a shim. Maybe the new pics will make it clearer.

As far as 1/4" mud goes, these pics are like 1/100th of the job.

Here is a link that might be useful: more pics

    Bookmark   June 9, 2013 at 6:39PM
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sombreuil_mongrel

You can easily substitute 1/2" plywood for drywall in cases of need.
I made my arch out of plywood, mudded it to the old plaster with durabond, and it worked out fine. From 2008 until today, in a bathroom.

In my experience, it can hold up in a humid bathroom for at least 5 years.
Casey

    Bookmark   June 9, 2013 at 7:02PM
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tbo123

Thanks Casey.
Looks nice.

still trying to find a supplier for Durabond locally (S. Florida).

What climate are you located in?

    Bookmark   June 9, 2013 at 7:30PM
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