I'm working on an 8ft x 12ft climbing wall. I have 3/4 inch ACX plywood attached to a frame. Since the frame isn't perfectly square, the plywood overlaps on the edges (up to about 1cm) so I need to shave it down. What's the best tool for the job?
I was thinking of using a block plane, but I've heard it can tear the edges of the plywood. Would a belt sander be better?
A block plane might work but you're going to have a very tough time taking down 1cm of wood over such a long length. If you do use a plane, go in small increments and make sure you keep your blade as sharp as possible.
A belt sander will work better, provided you use 40 or 60 grit sandpaper. Anything higher (smoother) and it will take you an eternity. My biggest caution with a belt sander is to be careful to work evenly. It's easy to get distracted and create a divot where you've sanded too much.
If the sanding is daunting or you want something with more precision, I would recommend using a handheld router with a flush trim router bit. This will get you a nice clean edge. I would try to get the board trimmed down to less than 1/4" overhang or so first if possible, with a jigsaw or something. Then clamp a long straight piece of wood along the line you want to keep.
If there's enough of an overhang to get enough stability for a circular saw and a guide, that would also work. But it sounds like the overhang is too small for that.
Suppose the black square is the area to be planed from the plywood, then cut near the 4 edges of near the square with 45 degree angle (2 bottom edges is done for reference in the attached figure) with a hacksaw blade or whatever and finish with a hand file (not really necessary). Later plane across the square as usual. The edges will look almost good, however while nearing the square, if you shift to a small smoothing plane instead of a jack plane, the result will be 100% perfect.
Sorry for my bad photoshop work.
