How to keep Basswood from warping

Are you using Balsa wood or Bass wood?

Bass wood is pretty stable and takes stain well with little or no movement. Its what they use for intricate carving, wooden boxes, venetian blinds, musical instrument components, plates and some architectural elements.
While its grain is somewhat noticeable it is pretty smooth and clear.

Balsa wood is a soft wood that you see in alot of the hobby and craft markets. It has several types of milling grades and the most common (like the sheets we see in the hobby shop for doing diorama work etc.) are "A" Grain. This is usually milled into sheets and has long fibers that show up as long grain lines. It is very flexible across the sheet and bends around curves easily. This flexibility and the way the grain runs makes it warp very easy. It will have a "soft" or velvety feeling to it.

Warpage is usually the result of wood drying at different speeds along the grain causing one side to contract or swell fast than another. This can result in warping, skewing, chequing, or even splitting.

On another note, regardless of which type of wood you used,
Did you only polyeurathane one side or both? (one side only will not have grain movement on that side, with grain movement from moisture absorbtion on the other, the resltant tension will make your wood happy)
Did you eurathane the ends too? (A place for moisture to absorb. its why furniture wood suppliers wax the end grain of hardwood lumber)
How may coats of eurathane did you use? (Balsa and bass are both "open" celled woods, meaning that the cells lose their water and this leaves large celular voids that give the woods its rigidity as well as its lightness. Fail to fill these up and they will absorb moisture differently)
How dry was the eurathane before you added your spackle? If it was still damp, then moisture can get through.

If you are "spackling" over bass or balsa wood to make a wall for a building, you be better served by using something like foam core, gator board, foam sheet, or even thin ply, ply wood to use as a wall core. these are more stable and you won't have nearly the trouble with warping.

I am using basswood 1/4 thick. I urethaned both sides and ends, left it hanging to dry. Switching to plywood is a good idea except that I already bought and cut enough basswood for 18 multistory buildings (3x8 foot Somalia Dio.) What if I brushed the ureth. on one side, then left it lying on a table to dry, then did the other side??

You could try using Thompson's Water Seal Before Ureathaning, see if that may curb the warping...

if you are using balsa wood any grane A B or C, i like to use Shellac clear spray on both sides of my balsa project, this will seal the wood beautifully not allowing any moisture to enter and warp your wood. i sprayed on the tail feathers of this dlg glider

i have flowin in damp weather, even in the rain and no warping at all.

hope this helps.

PS: you might need to use 2 coats if your balsa wood is super thin say 1/32nd sheeting

Ypu may have put it on too thick.  I always start by putting on some thinned stuff, sand that when dry, and thicker coats later.  I am surprised that 1/4 stock warped, but for thinner wood I clamp it down to a bench (or stick it to the bench with double sided tape) then paint it one side at a time.

 

Holy dead thread revival! You guys do realize the original thread (and last one) are from 2005? I doubt it is still an issue for him. It is always a good idea to check the date(s) before replying to thread you have searched for.

Back to your belltower, Quasimodo!

But seriously, it's kind of fun to hear from a flyer once in a while.

 

Bill