I would call them and explain what you want for a final project, what are your expectations, see what they recommend for painting over their product. They may tell you that shooting Omni straight over their coating will give you what you expect, who knows until you talk to them. My personal view is this, the coating you have now has a texture, and it sounds like some kind of tough coating that is kinda' slippery, it will be very difficult to sand it enough to get down into the divots and give the new paint any "bite". Just my 2c and I would not want to go through all that labor just to have my new paint peeling off with a pressure washer.
I agree with 78. I'd use a non sanding sealer primer first. Prep the existing finish like you normally would n preparation for shooting the top coat. Shoot two coats of the non sanding S/P. While you're cleaning your equipment & mixing paint & whatever, s/p will be curing (usually 30 to 60 minutes) & you'll be ready for topcoat
Sealer/primer , besides better adhesion, gives a really nice uniform base for the top coat. That's my .02 worth.
Sorry, just read the rest of the thread. Try sanding with 180 dry for the first round of sanding, then switch to 320 prior to the non sanding sealer/primer. Good luck.
