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.
