What went wrong and how do I fix it?

[zootroy]

The top of my desk return is bowing.
This is what I did:

1. stacked and sticked the maple in my garage for 12 days. Temp between 20 deg and 40 deg.

2. jointed and glued without planing (S2S). Alternating growth rings on glue-up.
3. Took it to the drum sander the next day. Two swipes on the bottom at 40 grit to get it cleaned up. Multiple passes on the top to 120 grit to get it flat and smooth. Overall, 1/8" removed.
4. Next day ROS sanded the top to 220 and applied first coat of wipe-on varnish (50-50 poly, mineral spirits). No varnish on the bottom, only the top.
5. Continued to wipe on varnish for the next two days.
6. looked at it this morning and she was bowed about 3/16"

Questions:
1. What are some of the possible causes?
2. How do I fix it? I was thinking of building a press, or can I straighten it when I screw it into the frame?

TIA for your suggestions and ideas.
Jason

[sacherjj]

"no varnish on the bottom" Here is what that does: The top is now pretty well sealed to moisture. The bottom, however, is open to absorb whatever moisure it wants. When wood absorbs moisture, it expands. This makes the bottom get longer and causes the assembly to bow upwards. Just a theory, but I have seen many a board bow on the non-finished side.

If that is the case, then you need to dry it out and put finish on the bottom. (Perhaps lay it out in the sun to dry out, I don't know.)

[scorrpio]

Not only that, but that 40 grit on the bottom
1. cut down to 'inner wood'
2. wide opened grain pores.

So, you sealed one side to moisture, while pretty much turning the other side into a sponge.
At this point, I'd leave the piece bottom up, with something heavy placed in it center, in a real dry place. Hope it'll flatten out some. Then, sand bottom to 220 and seal it.

[JimD]

The bow is gentle enough that you should be able to flatten it when you screw it down. Make sure you only fasten one side rigidly, however. You have to allow for wood expansion. You can use slotted holes on the other side or buttons. I would finish the bottom first, however, as has been recommended.

I do not alternate growth rings. I think that is an old wives tale. I match up the grain and glue up ignoring the orientation of the rings. I am sitting in a room with three table tops made this way that are all flat (and all around 2 feet wide finished on both sides).

Jim

[just4funsies]

Definitely dry the wood and straighten it out BEFORE you seal it, or the moisture that got in won't be able to get out. See how much of the bow you can get out by drying before you try to spring it straight.

[zootroy]

Update on desk tops

Thanks for the info and advise everyone.

I had two returns and on desk top. The desk had not warped, so I let it sit in a dry room for a few days, noticed no movement, so I sealed it. The returns are weighted in a dry room and leveling out. I'll chalk this up as learning a valuable lesson.

Thanks again for your help.
Jason