We have a standard park bench (from Lowes) that has been sitting in our backyard for some years, and it has acquired a pretty severe warp on one of the boards on the table top, and one of the boards on the bench. We wanted to restore it so the first step was to see if I could get the warps out.
Step 1: Clamp the Boards
You will need some strong and wide clamps, so I purchased some Irwin "Quick-grip" clamps at lowes. I also had some old (like 1950 old, from a wall I removed in my home) very straight strong 2x4s around (it pays to hold onto some things.. :).
Using the 2x4's and the clamps, I was able to start the un warping process by clamping the 2x4's to the top of the boards. The warp was going up on either end of the boards, so the clamps pulled the boards up in the middle; IE the 2x4 was on the concave side of the warp.
At first, I was not able to fully straighten the warp...
Step 2: Moisten the Boards
After clamping, I flipped the bench upside down and placed wet towels on the warped boards (this is the convex side of the warp). I placed it in the sun and periodically re-wet the towels every day over the next week.
Step 3: Tighten the Clamps
Daily, I would tighten the clamps. After a few days I was able to get the clamps all the way closed and the warp mostly straightened. After sitting in the sun for a week, I removed the clamps, and viola! The warp remained gone (mostly..).
To fully remove the warp you should place some wood spacers at either end and continue to clamp down until the boards are actually bending the other direction from the original warp. When you remove the clamps the boards will spring back slightly, hopefully to a normal shape!
Step 5: Restore
After getting the warps out, it was time to sand and paint the bench. But that's another instructable!