Is it normal for a cab door to curve when painted one side?

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Forum topic by jtcarn posted 03-23-2017 04:19 AM 1100 views 0 times favorited 4 replies Add to Favorites Watch
View jtcarn's profile


12 posts in 970 days

03-23-2017 04:19 AM

I’m painting the last 2 of 32 doors. Target lacquer on poplar doors with MDF panels. After spraying the back sides tonight, I noticed something odd. The 33” doors are “curving” away from the painted side. The glue up was flat and all my other complete doors are flat. Is this just from the wood soaking up the finish on the back side? It Freaked me out because these doors are remakes from 2 stupid mistakes. I’m guessing they’ll come true when I finish the fronts tomorrow?

4 replies so far

View Madmark2's profile


403 posts in 785 days

#1 posted 03-23-2017 01:51 PM

This is why you finish both sides ar once.

Yes the issue is humidity based. Now the trick is to get them back flat before you finish. Spray a little water on the concave side and lay that side down for an hour to see it it recovers. If you finish the other side while the piece is warped you’ll set the warp.


View jtcarn's profile


12 posts in 970 days

#2 posted 03-23-2017 02:26 PM

Interesting. I hadn’t noticed this yet, but I always did 4 coats on the back and then flipped and did 6 coats on the front – pretty much right after I did the backs. Thanks for the info. Chalk it up to the learning process. Just like I learned that a corner cabinet doesn’t get a 1 1/4” overlay like the others and always make sure you’re drilling handles in the right door!

View JBrow's profile


1366 posts in 1117 days

#3 posted 03-23-2017 02:46 PM


I am a little confused; I confuse easily. But my thinking is more or less in line with Madmark2’s thinking and so my comments mostly amplify Madmark2’s comments. The concave side of the door has less moisture than the convex surface. Therefore by either increasing moisture on the concave side or decreasing moisture on the convex side by just the right amount, the door should go back flat. Lacquer is applied to the door while cupped or bowed will lock in the moisture imbalance and it could be a long time, if ever, that the door would go back flat.

I assume the door is a frame and panel door where the frame is poplar and the center panel is ¼” thick MDF. I also gather from your description that if you were lay the door on the workbench with the back (finished side) down, the door cup/bow is crowned upward (convex side faces up). If this is correct, then some moisture has entered the unfinished front after lacquer was sprayed on the back. Alternatively, perhaps there is a property of the lacquer you used that acts as a desiccant and pulled a little moisture out of the backside wood. I am doubtful of this alternative explanation, but since I have not used lacquer, I cannot say for sure. But then if the cup/bow developed very soon after the lacquer solvent flashed off, I would lean toward this explanation.

In any event, if I have described your situation accurately, the drier side of the doors has lacquer applied. This means applying moisture to the lacquered side (concaved), as Madmark2 suggested, would have little effect since moisture would have a hard time making contact with the wood. If this is the case, then the door could be placed in a box with the lacquered side down. A 100 watt light bulb mounted at the top of the box and shining on the convex unfinished side of the door could be enough heat to drive off just enough moisture to allow the door frame to return to flat (I doubt the MDF, on its own went out of flat). If you go with this drying out method, keeping a close eye on the door is important. Otherwise too much moisture could be driven from the wood and the cup/bow could reverse.

As to why this happened on only 2 or 32 doors is difficult to say. Lumber milled from a log in different ways (quarter sawn, rift sawn, flat sawn) can move due to moisture in different ways. Also, I know here in the mid-west, the weather is changing quite a bit throughout the day and from day to day. The humidity has recently gone up by a good bit during the daytime and drops at night. Perhaps the changes in the weather have had its effect. As Madmark2 said, the best insurance from this happening in the future is to get finish on the both sides doors as quickly as you can.

View jtcarn's profile


12 posts in 970 days

#4 posted 03-24-2017 01:30 PM

Wow, that is quite a response Jbrow – thanks. I tried the water-down trick and then sprayed the fronts. They came back true(ish) but a lot better than they were. Job complete, panic over.

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