What happens if you run painted lumber through your planer? That depends on what the boards are painted with, and on what's hiding under the paint. In any case, it'll dull the blades. Here are two opinions on the practice, courtesy of our friends at the Woodworker's Journal:
Q: I was given a 12" planer for Christmas. Is it OK to run painted oak through it? I've double-checked to ensure there are no nails or screws in the lumber, but wonder if the finish on the wood will dull the cutters.
A: Carol Reed: Yes, it's OK, and yes, it will dull the cutters, but then everything you run through the planer will dull the cutters. That's why they have to be sharpened from time to time. Given the mess of stripping and the expense of the strippers, I'd plane it. The cutters in the planer probably need sharpening anyway. At least you'd end with sharp cutters in the planer as opposed to a toxic mess to dispose of. Life is a series of tradeoffs, isn't it?
A: Cal Brodie: Several years back I received a stack of wood covered with latex paint. Yes, the paint dulled the blades; no, it didn't ruin them. I just had to sharpen them when I was done. The call to make is whether using the wood is worth the cost of blade sharpening. If there is a chance that the paint may be lead based, I'd be cautious about any exposure to the dust and might choose to pass on planing the boards.
From the Woodworker's Journal eZine archives
Painted wood dulls planer blades rapidly because paint contains abrasive minerals. If you are really attached to certain pieces of painted lumber, a set of dull blades may not be a huge issue. A bigger concern is the fact that painted (and therefore used) lumber is notorious for hiding metal fasteners that can ruin your planer blades and seriously damage your planer. There's no shortage of horror stories involving used lumber and mangled planer blades (and worse). They are often told by the hapless planers of painted wood who were sure their reclaimed treasure was "absolutely clear of metal".
One way to make planing painted wood safer is to give it the once-over with a metal detector designed for checking lumber. The Lumber Wizard 4 will do the trick. Some woodworkers believe that it's safer and more effective to run painted lumber through a drum sander, such as one of the Performax models. If you check the lumber over thoroughly for metal and other foreign objects before you run it through, damage to the sander itself is unlikely. The piece of mind may be worth a couple of replacement 36 grit sanding strips. Finally, if you suspect that the paint contains lead, take care to protect yourself from dust and fumes during the process, or better yet, forget about it and buy new lumber.