Q & A: The Chalk Test

Q: How can I spot flaws in my wood’s surface before putting on a finish?

A: Try this classic method: after jointing or planing, lightly rub the wood with the side of a piece of chalk. Chalk colors all the high spots; low spots remain untouched. You may see flaws that are very hard to spot with the naked eye. Here are some examples:

• Planer and jointer snipe. When a planer takes a deeper cut at the beginning or end of a board, it’s called snipe. Snipe can be a real pain to sand out because it may be up to a paper-thickness deep. Chalk makes snipe stand out like a sore thumb. Snipe from a jointer indicates that the outfeed table is set too low. Raising the table fixes the problem.
• Jointer milling marks. Jointers with three-knife cutterheads always leave a small ripple pattern at right angles to the length of a board. Each knife creates a small crest, like a wave, and a trough. On a final pass, these crests should be fairly close together in order to reduce sanding time, where you have to get down to the bottom of all the troughs. If one of the knives is set too low, some of the crests will be quite prominent; if you’ve pushed the stock too fast, the crests will be too far apart and the wood’s surface will look like a washboard. Chalk quickly reveals improperly set knives and washboarding.
• Nicked knives. A nick in a jointer or planer knife creates a long ridge down the length of a board. Chalk highlights the ridge. On most planers and jointers, you can shift one of the knives sideways to temporarily eliminate the ridge, but usually this is a sign that your knives are getting dull.
• Hand plane marks. If your hand plane’s iron isn’t adjusted exactly level with the plane’s bottom, it will dig in on one side and leave ridges. Chalk makes these ridges easy to see.