^ like you stated painting houses isn't close to painting cars. Thats apples and oranges. When I was in school learning auto body and painting, the painters that came in claimed they could paint a car, 1 tried shooting a hood and it looked like complete shit. And 25 PSI won't be close to enough pressure to spray, you'll need over 100+ PSI constantly depending on the gun your using. but like I said i'd buy enough paint at 1 time, because even if its the same exact color, it could turn out different if bought at different times.