Stair tread cupping during refinishing process

[Garry Smith]

Last weekend I made up 12 tread and risers for a stairway using ash. Last night I was putting on the 2<SUP>nd</SUP> coat of polyurethane varnish (in the shop) and after I had finished the varnishing I notice that all the risers and tread started cupping. I used sanding sealer on the back sides of some of the treads and none on the risers and all are cupping. I cannot stack them because of the fresh finish. The past 2 days have been very humid. If anyone can give me any advice on how to flatten them I would greatly appreciate it. Also I need to apply one more coat of varnish on the top side.

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Garry

[Donnie Raines]

Garry,

A couple of things:

Did you apply finish to both sides of the treads?

How did you store the treads after milling them?..after applying the finish? You need to have air flow around all sides of the board to ensure correct acclimation of the boards.

Do you know how these boards were dried?

Can you see the pith of the log these boards were cut from? Meaning, when looking at the ends of the boards, do you see the growth rings make a half moon shape at all? If so, thats part of the issue(maybe not all of it).

[Garry Smith]

Some of the treads do have sanding sealer on the back side and some do not. All of the treads and risers cupped regardless if they were sealed on both sides or not.
The wood has been kiln dried and where very flat for a week or so after i rough cut them to length but i did have them stacked. I am varinshing them in the shop laid out on long 2x4's and saw horses.

[Jason Roehl]

Sanding sealer isn't going to do much to keep out humidity, certainly not to the degree that your varnish is keeping it out. The bottom line is that if they are cupping, there's more moisture on the outside of the curvature versus the inside. If it was high humidity, and you slathered on some varnish on one side only, you sealed it up real good, much more so than sanding sealer could ever hope to. You may have to heat 'em up somehow to get some of the moisture out, then varnish that other side.

[Tim Sproul]

Hmmmm....

I finished my treads AFTER attatching them to the stairway......Haven't moved in 2 years :)

[Garry Smith]

Thanks,

I'll try the heating trick to see what happens, I have a radiant heat lamp that may do the job.
Garry