as far as prep goes. I usually clean anything oily or greasy in the parts cleaner, then wash in degreaser. old paint is either removed by media blasting or paint remover (I almost never use paint remover). if the old paint is well adhered and doesn't have gouges and scratches that I can't completely sand out, I might leave it and just sand. typically, I don't sand smoother than 250 or 400 grit. want some "tooth" for the primer to grip.
if blasting to bare metal, you want to prime fairly quickly after to prevent flash corrosion. if painting bare aluminum or magnesium, I do an Alodine wash and rinse before priming.
I always use a tack cloth before spraying primer or paint. when I need to do a solvent wipe down, I use acetone. mineral spirits or paint thinner is actually a very light oil, so keep that in mind if you use it to wipe down before spraying. won't hurt if you're shooting alkyd enamel but dunno. opinions on this vary.
sanding is the work. if you have any low spots in the old paint (like afore mentioned gouges), they will look like craters in the primer unless you sand them completely out to a very long taper. if the old paint starts flaking when sanding then it all needs to come off. primer will show every defect. if you have any defects then start over. do not shoot the topcoat. be picky. why go to all the trouble to do this if you're going to have defects? I do use high quality light weight body filler and then spot putty to fill voids in metal, esp if I blast castings. filler does not work well for getting rid of gouges and scratches in the old paint layer though. they need to be sanded out.
painting is a learning process. I'm self-taught, which means I had to do a lot of things over after effing them up in the beginning. there are really only 2 settings on an automotive type spray gun, fan and material. the fan knob can give you a wide fan or a small round spot by turning the knob. the material knob can be turned to spray nothing to flooding. a wide fan is good for large areas, a spot is good for narrow work, say the pickets in a gate. if you have the material set well for a wide fan and then set the fan to a spot, you will flood the work and have a huge run. so, if you change the fan, you must change the amount of material coming out of the nozzle. oh, there are different orifice nozzles for different paints. I always have a large sheet of cardboard set up near what I'm shooting to set fan and nozzle. if the paint comes out looking dry, you need more material, if it floods then less, of course.
if you are spraying and get a run, STOP. do not try to fix it, you will fail. do not wipe it off. let the work set up until you can sand it off and do it over. several light coats are better than one heavy coat when you are learning. to get good gloss, you need to lay on enough paint to get some flow, just not to the point of running. if you make mistakes, don't give up.