woodwoodworking
I have two sections of 3×12" wood that have been out in the weather for a few years and are now bowed across the 12" width of the wood. The dip is pretty severe, maybe 1/2 an inch? It seems like sanding would take way too long.
What would be the easiest way to flatten them out again? Is there a tool designed to do something like this?
One of the best tools to begin the process of removing old, thick paint is a simple hand scraper
I would remove the metal strip, if possible, and use the scraper to remove most of the finish. The edges on the scraper can be rotated as they get dull.
Then you can follow up with either stripper (there are new citrus based that are pretty effective if you let them sit), or an orbital sander.
If it is very resistant, you could use a multitool with a scraper blade or even the grinding plate if it is very hard. This is an example, but there are many simpler and cheaper versions.
Caution with the multitool because once you cut through the paint, the wood may gouge or tear.
For trim work that will be painted, use paintable caulk instead of wood filler. It does not crack and it is very tolerant of shifts in the pieces, both due to structural movement and seasonal swelling and shrinkage.
Wood filler is better suited to holes and divots fully contained within a single piece of wood.
Best Answer
The power tool you would use to take the board down would be a thickness planer:
It has an adjustable cutting head and you run the board through it in successive passes to shave off the surface.
However, unless you do a lot of rough lumbar milling or are into fine woodworking, it would probably be silly to buy one. Maybe you can rent / borrow?
Note that a thickness planer is good at making 2 faces parallel, but it won't be much help if the wood is also twisted or bent along its length. The twist will just follow through the planer and you will end up with a twisted board with 2 parallel (but not planar) faces.
Edit about the suitability of thickness planer:
Several people have pointed out that to use a thickness planer you are "supposed" to have one face flat already. I guess that's optimal but I have used a thickness planer many times to "save" cupped boards. If you have a great piece of wood, you might as well try something, right?