The more durable "paint" found on most folding bicycles today is actually powder coat. This is a plastic which is bonded to the frame with heat. It won't bond to paint, it needs to be applied to a bare frame. Most powder coaters can also strip your existing frame for you.
Around here (Seattle, WA) a basic one color powder coat job on a normal bike runs about $100.
No spray paint or anything home applied is as durable. I use spray paint when I'm touching up repaired areas on frames, but send them out for powder coat if the whole frame needs paint. Powder coat is also much more environmentally friendly than paint, there are no volitile organic compounds used to apply it. The powdercoat is applied with an electrostatic charge which holds the plastic beads to the frame until it is melted in an oven.