HotRod wrote:
If you want to do a lot of mudding in one weekend start with some sheetrock.
Sheetrock 60 or 90 will dry really fast and is a little harder than the mud
you'd use. It's not designed to be used for the finish coat.

Are you referring to Durabond or Easy Sand setting type compound?
Durabond can't be sanded and shouldn't be used for the polish coat, but
Easy Sand works just fine. It'd be less expensive and less labor to
use ready mix joint compound, and since the OP's doing a large garage
it's doubtful he'd be able to finish the first coat in a day anyway.

R