Warping of a Tabletop with an Unbalanced Finish

A teachable moment: Finishers explain why tabletops finished on one side only (and in humid conditions to boot) experienced bad warping. December 31, 2005

I am in the middle of building 38 table tops for a local restaurant in my area, Lake Oconee Ga. The tops are made of Euro Beech, 1-3/4 thick, glued up from 4 to 6 inch rips. There is a 3/8 x 1" inlay in each top which is 5 inches in from each edge running the width of each side. The tops, half of which are 30" x 30", and the other half 42" x 42", are stained burnt umber and finished with 2 coats Krystal on the top side only. The bottoms of the tops are just stained. Prior to spraying the Krystal, the tops were perfectly flat having been sanded on a wide belt, and then hand sanded to 150. Now after sitting 3 days the tops have slightly warped across the grain. Is it possible that tension created by finishing the top and sides only caused the warping? Do you think it would help to go ahead and spray the bottoms also to equalize the tension? It seems like I read on this forum once that this could occur across a wide surface. It's not severe on the 30's and I can probably get by with it, but my concern is the 42's. I would greatly appreciate some advice if you think that surface tension could be the culprit. If any of you are millworkers, do you think that the 3/8 x 1 dado cut for the inlay across the grain could cause the tops to warp up across the grain? Should I cut some kind of relief in the bottom to try and equalize the tension?

Forum Responses
(Finishing Forum)
From contributor A:
Yes, only coating one side of the tops with Krystal caused the tables to warp. It did this in two ways. First, you sealed one side which leaves the other prone to absorb moisture from the air. This will cause the table to warp. It should be concave on top and convex on the unfinished side. Second, when a pre or post catalyzed finish dries it shrinks, and has enough strength to warp the table, again in the same direction. You may be able to get rid of the warp by placing the tops on a heated surface to drive the moisture out of the bottom of the tabletop. After it is almost flat coat the bottom with the same amount of coats you put on the top. When this dries it should pull the table the rest of the way flat. This is going to be a hit or miss problem, because I don't know if any excess moisture has caused the warp or if it was just the shrinkage. I suggest you take the one with the least warp and just give it a spray of Krystal (same coats as the top) and see if it reverts to flat. Give it a day or so to shrink and see if it flattens out. If it doesn't go all the way flat then you have a moisture problem also. Sounds like a hard knocks lesson has been learned here.

From the original questioner:
I suspected as much. The humidity in this area was extreme last week after the moisture from Rita settled over the south. I kept the raw tops stored in my finishing room with a humidifier running constantly. Fortunately, I've only finished 12 of the tops and hopefully I can flatten them back out and spray 2 coats on the backs and hope for the best. Should I spray the sides also? They already have 2 coats and I don't want to get the finish too thick. I'm going to leave them stacked face to face overnight on the flat surface of a table saw with some weight on the top to try and force them back to flat. Then I will quickly spray the backs tomorrow if it works.

From contributor A:
If you sand the edges lightly (320 grit) and spray them once more you shouldn't run into any problems. You may want to mask off the top so you don't get any overspray on them. Spray the edge on your second coat of the bottom. If you have an electric heater you could lay the tops on horses and 2 x 4's and put the heater underneath (raw side down) to speed the drying up.