I've never sandpapered parts before, just sandblasted. I think sandblasting removes more oil and impurities that could cause problems in the powder. A sandblasted part is bare white metal, where sand paper leaves a lot behind. You could try it, but I don't know offhand how well it would work. Try on scrap metal first. Getting the corners, nooks & crannies would be very difficult with sandpaper. A benchtop sandblast cabinet at HF sells for $125, but you need a compressor too.
It's easy to spend a bundle of $$ on a trip to Harbor Freight
Hemi-T
BTW, don't buy your powder at HF. They get $9.95 for a 8 oz can ($19.90 per lb!). Caswell powder is much more reasonably priced, and comes with this cool forum to help you use it
