What is Warp?
First of all, let’s talk about what “warp” is and what it is not:
Warp is a bend or “cupping” of a board across the grain, not with the grain. Usually this means that a board will be warped across the width of the board, not its length. (When a board is bent or cupped with the grain it is known as “bow.”)
Causes of Warp
To understand warp and similar problems, we need to know what causes them. Warp is typically caused by uneven humidity. For instance, if a board is laying on a flat surface and its top surface is unprotected, that surface will adjust to the humidity in the room by either taking in or releasing moisture.
If a room is very dry (usually in winter months) that side of the board will dry out, causing the cells to shrink, thus causing the width on that side to bend. If a room is humid (summer months), the board is bound to suck some of that moisture in, causing that side of the board to expand. So if you have an exposed board that is not warped yet, cover it!
Solutions for Warp
Now that we know what warp is, let’s discuss how to fix it. There are several methods of fixing warp:
- Lean the warped board up against a wall with the cupped (concave) side facing the wall. Let the board remain in this position overnight or longer. If this takes care of the problem, sand and finish immediately—or, at the very least, wrap the board in something to protect it from the air. Don’t try this if your room is very humid.
- If the warp is not too pronounced, you can steam it out. Lay a damp towel on the cupped or concave side and heat it with an iron. The steam penetrates quickly into the wood fibers, causing the cupped or “short” side to expand. Move the towel and iron up the length of the board until it appears flat. Clamp the board between two straight boards so that it will stay flat and air can circulate fully around it until it is completely dry.
- Another way to remove warp is to apply water and heat to different sides simultaneously. Start by wiping the entire concave side with a wet towel. (Be careful of how much water you put on the board: if it is a glued up panel, copious amounts of water could weaken the glue joint, so go slow!) Now lay the board on a flat surface with the wet side down. Apply some heat to the other side of the board with a heat lamp, or just weigh the board down in the center and let it sit until dry.
Any time you wet a board, just remember that you’ll have to sand it thoroughly before finishing to remove any raised grain or stains!
May your path (and your boards) be ever straight!