For many years I pressure washed concrete floors the day before epoxy coating with 6,000 psi via a rotary jet that looked like a lawnmower and never had an adhesion problem.
I spent time with a wet vac to remove any free water from joints etc. afterward and the next day it was always white and clean and dry. If there was oil in the floor which was often on the jobs I specialized doing, I would spread a strong water-based alkaline cleaner over the floor, brooming it in and using a ride on scrubber for larger floors, vacuum it up, then rinse and vacuum, then rinse and vacuum once more, for larger floors we used a fire hose for rinsing. Next day it was always white and dry.
I found that you have to use 3,000 psi plus to remove the acid etch white residue and the jets have to be close to the floor.
I found that 6,000 psi did not penetrate more than one sixteenth of an inch and that moisture in concrete was from the bottom up and slowly wicked to the surface over a long time, you cannot force water in quickly which is why pavements dry quickly after rain.
If you wash you have to clean out the joints thoroughly or else you will have wet mud sitting there and the concrete along the joints will remain wet for a few days.
Diamond grinding is dry and better, but I always washed, broom agitated and wet vacuumed after diamond grinding to remove the dust. You could then eat off the floor no problem it was so clean.
These were not garages, they were industrial plants with forktrucks whizzing in and out 24/7 with no adhesion problems. Just two coats of 100% solids epoxy.
