I've always painted outside. Do it on a nice summer day, early in the morning. Wet down the area the night before, put down a tarp and put the car on it first thing in the morning. Shoot the paint before it gets warm, while the air is still calm and no bugs are buzzing around, let it cure for an hour or two, strip the masking and put it back in the garage to cure. I've done a couple this way, and they have all turned out nice. The ones I've seen done inside always have some serious overspray to deal with after the fact, and even know one guy who spent a couple of days in the hospital because he did it inside without a resperator (and his paint looked like crap!). Lots of fans help, but you will still get overspray, both on the car and all over your garage. Good luck!