Troubleshooting Wood Cupping when Finishing

Unbalanced wetting of solid wood surfaces can cause the wood to cup. October 28, 2009

Question
I am in the middle of finishing an edge glued pine top 21"x52"x3/4". So far I have dye stained (water) top and bottom, one coat of General Finishes black acrylic paint around perimeter of bottom side and two coats on the top side. When I finished yesterday the top had cupped severe enough that it was easily noticed. This morning it has flattened back out about 95% to where it is acceptable. Now I plan to distress it lightly, glaze it (Generalís wb glaze) and finally top coat with Campbellís Aqualente. I am concerned about the cupping coming back. Should I even out the coats top and bottom with the black as well as the clear coat or is there a better way?

Forum Responses
(Finishing Forum)
From contributor B:
There are two things you can do. You can put braces across the bottom, going cross grain, or use pine plywood. I would use the plywood, end of problem.



From contributor R:
Itís best to apply just as much finish to the bottom of something that you do on the surface. It has something to do with balance. I like the idea of installing braces on the bottom of the top. You didnít mention if the top was solid stock or an MDF core or a plywood core. It does make a difference in how the wood will behave.


From the original questioner:
It is solid pine, 2" to 3" strips edge glued together.


From contributor R:
Did you reverse the grain on each board? You have between 7 and 11 pieces of wood that are glued together and there is a lot of natural stress going on.


From Gene Wengert, forum technical advisor:
The only thing that makes pieces warp or change size is that the MC is changing. So, the piece had a wet side that caused that side to swell. The swelling forces are very large and are hard to hold flat. If you do use a brace (I suggest you avoid it as you are asking for trouble), the screw holes must be slotted to allow the top to move as the MC changes month to month (or when uneven finishing is done).

The solution is to prevent uneven moisture, top and bottom. Then you will not have this cupping. Note that the idea of alternating the grain of individual pieces is often given as a solution, but the benefit of such altering is very small. Wetting one side and not the other with a WB finish will create such a difference, face to face, that the piece will be unsatisfactory even if the grain alters. It is really rare, especially with softwoods that do not move much compared to hardwoods, for commercial operations to alter. They know that if the MC is OK, then this extra step is a waste of time.