Water-base Causing Warpage?

[Harvey Pascoe]

Anyone else have this problem. particularly with thin wood? I make boxes which sometimes have tops of thin solid wood 3/8 to 1/2" thick. The top is coated with varnish, the underside water-based. Some wood has more stress in it than others so that occasionally the top warps after applying the water base. Sometimes, after curing a week or two the top will spring back to flat, sometimes not.

One partial solution I found is to make the first coat very thin, when spraying just a light dusting, or when brushing lightly scrub with a nearly dry brush until the wood is barely wetted. This is not fool proof as I just had one that warped.

Anyone have other solutions?

[Scott Holmes]

Adding oil based finish to one side and water-borne to the other is most likely causing a difference in the amount of water vapor or humidity thus causing the warp. When it dries out it may or may not go back flat.

Does it cup toward the oil based side? WB ( OB
Waterborne finishes are not as water vapor proof as oil based finishes.

[Howard Acheson]

Scott has the reason. One solution is to apply a coat of dewaxed shellac to the underside before applying your waterborne. The shellac is a very effective blocker of water and water vapor.

[Harvey Pascoe]

Does it cup toward the oil based side? WB ( OB
Waterborne finishes are not as water vapor proof as oil based finishes.

Hah! I knew that but never thought of it. The crown is toward the WB side, but in my case I think the water transfer is happening at the time of finishing as that is when the warping first shows up. Not severe but even 1/16" is too much for me.

[Steve Schoene]

Scott has the reason. One solution is to apply a coat of dewaxed shellac to the underside before applying your waterborne. The shellac is a very effective blocker of water and water vapor.

For that matter shellac alone may be a good material for the interiors. It contributes no long term odor and as Howard points out is a very good barrier against water vapor.

[Howard Acheson]

>>>> For that matter shellac alone may be a good material for the interiors

Thanks Steve. I meant to make that point.

[Bob Wingard]

UMMMM ... lemme' see here ... you're sealing one side of a piece of solid lumber ... then wetting the other side ... and wondering WHY it's warping ... sound about right ??? Even if the one side wasn't sealed, wetting the bottom can still warp the piece. Try laying a large panel that has no finish on either side on the lawn for about an hour. It will warp like crazy ... flip it over and it will flatten in about the same time frame. It's all about equilibration ... if both sides are equally wetted, the piece shouldn't move ... if one side is allowed to gain moisture ... it will warp.

How 'bout doing the waterborne side FIRST ... then sealing the opposite side ... see what that does for you.

[Jim Becker]

What Scott said...two different kinds of finishes on opposite sides of the wood can make for different shrinkage rates and other ill effects--water borne and oil based will stress the wood differently and you do get some water absorption by the wood on one side, too.. If you must to that, seal with de-waxed shellac first...it's one of the best moisture barriers (airborne moisture, not puddled water) and will help insure you have less issues.