I paint at a metal fab shop and do automotive painting at my house.Those are all good tips for a inexpensive paint job.I also use seran wrap to cover stuff.Anoth tip would be to spray bomb some clear over the paint to make it last longer.
I have had good luck with paint from hardware stores. Once you finish scuffing and sanding, clean the frame with glass cleaner or a mild solvent. Keep it clean until painted. You will be happy with it.