I would disagree with that. Use an 80 grit red garnet for a blasting medium and you'll end up with a better grip surface than sanding.
Best thing to do is buy your paint product, then see what that manufacturer recommends as far as surface prep, gun tips, pressures, etc. Or, don't buy the product but go to the paint manufacturers web site to gather all this info before you begin.
I would recommend this product.
Tamco Paint It lays on nice, is high build, and sands nicer than anything I have come across yet. Like butter.
Of course, if your going to leave your parts in primer for any amount of time, it would be better to epoxy prime it first, then work your fillers and high builds to prevent your rust from coming back.
