I've mentioned this on the blog and site before, but there is variance in manufactured products.
Homasote seems to be a constant repeat offender.
Ladies and gentlemen of the jury, I present exhibit #1 for the prosecution:
A different view of this exhibit:
This shows about 15 or 20 minutes work with surform rasps and coarse sandpaper.
As homasote is a paper based product, dust is absolutely retched.
The prosecution rests.