If you put the paint in the gun and little or no paint comes out you will know it needs to be thinned. For a gravity feed gun you would probably use it at about 60-70 pounds. I don't have any connection with rustoleum. I'm a professional furniture refinisher. I do use rustoleum paint from time to time.
I can't find anything to support the idea of spraying the paint in 15-20 minute intervals. I looked online for a data sheet and couldn't find one. The only thing I could find is a handling sheet which said what I thought all along that it was just an alkyd enamel. You might look on the can for drying times but if it were me I would not put more than one coat on in a 24 hour period. I think if you would put 3 or 4 coats on in an hour most of it would run off onto the ground. I don't know if you're old enough to remember but the paint is very similar to enamel house paint.