IMO you need to pressure wash after grinding to ensure the floor is really clean. Just scrubbing and using a garden hose is not going to clean out the pores as good as a PW. Especially if you let any of that dust get wet and then dry on the concrete.
What about the rollers? Do you have to change out the roller covers every ~30 min or clean them? I have 820sqft to do and I'm not sure how many rollers I'm going to need to complete it (primer, epoxy, clear).
Interesting read about your experiences with various machines. I'm also in the middle of prepping my floor for epoxy painting (Legacy's product) and having a hell of time getting the results I expected.
The local Sunbelt wanted $240 for a single head Edco with Dyma serts (3). For half the cost...