I thought plywood was very stable but not now. I made a bottom (22-1/2″ x 12-1/2″) to a cabinet out of 1/2″ exterior plywood, cut a 1/4″ deep x 1/2″ wide rabbet on it and put two coats of oil based poly on it. When the poly was dried I was suprised on how nice it looked, very flat and the fit was loose, but not to loose. The next day I went to do the doors and the piece had a 3/8″ bow in it. Next time I’ll make the bottom from solid stock.
Dave in Pa.
Replies
Dave, don't smack me upside the head if I'm stating the obvious, but did you only finish one side of the plywood? In other words, just the inside bottom of the cabinet, as opposed to both the inside and the outside, facing the floor?
Plywood is very stable, but it can warp if there are differences in the moisture/humidity exposure of both sides of the board. An example of this was the recent veneering thread, in which the initial poster veneered only one side of the plywood panel and found warp the next day. Veneering the other side (thus balancing both sides) solved the problem, evening out the moisture and expansion/contraction throughout the entire board.
If this isn't the case for you, then I'm going to pass the buck!
David
I did both sdies with poly, 2 coats. Still a mystery to me.
Still a mystery to me.
Hmmmm... me, too. Maybe the fact that it's exterior-grade plywood may have something to do with it. I'm just speculating here: but if I'm not mistaken, although exterior grade uses weather-resistant glues, the outer veneer faces also tend to be rougher and less-finished. That may mean that one side absorbed the poly at a different rate than the other. Perhaps a few more coats of finish would help balance things out.
Perhaps there's also less care taken in balancing the internal wood layers of exterior grade, with the manufacturer assuming that it will be nailed to houseframes rather than used for fine cabinet work. For construction purposes, etc., perfect balance may be less of an issue than overall strength and durability.
Finally, when I've been looking through stacks of plywood panels for my own furniture projects, I avoid warped panels like the plague. If they're already warped, I figure something must have gone wrong with the manufacture or storage... and who knows what it'll do once I get it to my workshop. (I realize that you said that your panel was flat when you started, so this last point is just mental vomiting.)
Anyway, I don't know from nuthin'... just some thoughts.
DavidHmmm... the garden or the workshop today?
Maybe the fact that it's exterior-grade plywood may have something to do with it. I'm just speculating here: but if I'm not mistaken, although exterior grade uses weather-resistant glues, the outer veneer faces also tend to be rougher and less-finished. That may mean that one side absorbed the poly at a different rate than the other. Perhaps a few more coats of finish would help balance things out.
You may have something there about the glue. I think I will bring it into the house for a few days and see what happens. If it flattens out I need to recoat one side.
Dave in Pa.
I'm with you. I cut out a bunch of dados in some oak faced plywood (for drawers). When I went to put in the dividers the sides were so warped that nothing fit. I hadn't put finish on either side so that couldn't have been the problem. I did probably wait to long between the time I cut the dados and when I got around to putting the whole thing together. But this is just a hobby and I don't usually have time to finish a project in one sitting.
It is perfectly normal for plywood to warp if you cut a long groove(s) or dado(s) in one side and then leave it exposed for a period of time. You have unbalanced the inherent strength in the plies.
Before we began using particleboard in the shop I was involved with we used plywood. Whenever we needed to dado or groove the panel, we would either finish up the glueup within a few hours or insert scrap spacers into the grooves if we could not complete the glueups right then.
I love this thread! I just learned something very useful about plywood which is my case building material of choice.
I have two 14"X48" hutch panels sitting in my shop with a rabbit and 3 dadoes running the width of the material. I'm going to cut the cross members and glue 'em up tonight! Thanks.
bill
It is perfectly normal for plywood to warp if you cut a long groove(s) or dado(s) in one side and then leave it exposed for a period of time. You have unbalanced the inherent strength in the plies.
Would rabbeting all four sides do this also? What has me concerned is that it is happening with 1/2" plywood, what will happen with the panels on the doors? I now have to elimate the rabbetsor make a 1/2" groove in the rails and stiles.
No, rabbets should not cause the problem.
Dave
I always use my moisture meter on cdx plywood because it can be ship and stored outside. I’ve had it be over 20% and I needed it 8% to 10%. The big box stores store it inside and sometimes it can be less, but not all the time. I could never use ½” ply because it always warped to much just sitting in the stacks.
Shop grade plywood is always flat and dry most of the time. I think this might be one of your problems.
Jeff in so cal
Thank You! Us weekend warriors still have a lot to learn.
Dave,
I assume, since the plywood is exterior grade, that you used construction plywood. All wood is going to be affected by humidity and temperature, and plywood is no exception. However, the more plies used, the more stable - generally speaking. Construction plywoods are usually built up using thicker/fewer plies than furniture grade. Moreover, the wood species used will also affect stability. Chances are your plywood was made from pine, one of the more vulnerable species as far as environmental change.
Next time, use solid wood if you like, but I think you'll find hardwood plywood a more stable alternative.
Edited 10/23/2002 5:06:07 PM ET by Jeff K
Were the pieces finished one coat/one side, then flipped to the other side, or both coats applied to one side first, then the other? Were all the bows facing up or down on the final coat?
I suspect an uneven curing problem. I let my box panels dry upright in salvaged dishwasher racks as soon as the finish won't dent so that it cures evenly, exposed on both sides the same. I seldom lose a box panel at this stage to warping. I use a lacquer finish, however.
If you have to treat plywood with kid gloves then why bother?
Whoever said that they were going to use solid wood for drawer bottoms... I say more power to you.
Doesn't everybody think it's time do rediscover the lost art of building furniture with real wood?
Edited 10/24/2002 4:48:17 PM ET by CHASSTANFORD
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