I've had sags and curtains before. You let them dry thoroughly and sand them with a wood block and 1200-2000 grit "Colorsand paper", then polish them out. It takes forever. You can't do anything while the paint is wet. It never works.
Biggest thing I learned this time is that lower air pressure is better for the home user. Most guns and lots of paints spec 90 psi, but they're using pro booths with exhaust systems.

By the time the job is done I'm so sick of it that I'm quite happy with the car being simply all one color. Be sure to get a good charcoal respirator. I also pull knit shirt-sleeves over my nose, mouth and head. Sweat builds up and you don't want that in the paint.