I just went through the pain of painting my plane for the second time. The first time the paint job really sucked. Here are the things that I learned and did. This is not the only way to do things, this is just my experience.
The first key is prepping the surface. Use a flashlight in low light at a really low angle, and all those little imperfections will just pop out.
Bought a good pressure pot paint gun. I have used several gravity feed guns, but painting upside down under the wings and fuselage really causes these to have issues. The first pressure pot gun I bought from Harbor Freight was piece of junk. Leaked from every connection. After several frustrating hours, I tossed it in the garbage, don't waste your money. (This was part of the first paint job being horrible)
The one I purchased and recommend is:
A good compressor is a must. I then put a pressure regulator on the output of the compressor. This may not be necessary, because the gun itself has an air output limiter, but I had it so I threw that on.
I then had to learn technique and tune the setup. I found a video online that talked about practicing with water based acrylic paint from the craft store. $10 a gallon, and water based. I bought some of that and added some water to thin it and got a bunch of cardboard to practice on. This worked out great, highly recommend this for anyone trying to learn to paint. Cheap, no noxious fumes, completely safe, and clean up with water. I spent over an hour spraying with different pressures and paint flow.
I finally settled on 15psi in the paint pot, and 30 psi from the compressor. The two controls on the gun had wide open air output and almost wide open paint output.
Used PPG paint and mixed according to their instructions.
The paint job turned out great. I still had a few dips and a sag, but hey, I'm an amateur. With a little light sanding and some wet sanding a a few places, it will look fantastic.
This worked for me... your mileage may vary.