I cut a square hollow from a thick (~20in diameter) pine log using a chain saw, which looks like this:
For my purposes, I don't need this very exact, nor very smooth, but I do want to get rid of those huge chunks of tear-out at the bottom. I've tried hand planing, but it's just not working; if the blade of the plane sticks out less than a tiny amount (maybe 0.2mm), it doesn't do anything, or quickly clogs with shavings, and if it sticks out more than that, it seizes in the wood and just makes more tear-out:
I've sharpened the planer blade, so I don't think that's the issue. The wood is probably not very dry, since it was just recently on the inside of a huge log, but I'm not sure if that's relevant.
What could be making this so hard to plane, and is there some smarter way of doing this job?
I checked the moisture of the cavity walls and it's about 15%. Is that too wet for planing?