Aerosol Spraypainting a surface that is already painted?

Question:

Aerosol Spraypainting a surface that is already painted?

I'm currently working with large stencils to paint a sign for a local business whose in a rush to get things done. The area which I'm suppose to paint is not a blank/unfinished surface (it's already covered in red industrial wall paint). I was wondering if it's wise to use Aerosol Spraypaints to paint on such a surface? I wish to use Aerosol versus regular wall paints because its easy and speedy, though I'm not sure if it'll hold up to the weather. Will it hold up to the test of time? If so, which products? What brand? What should I look out for? Some links of such products would be great help too. Thanks for your help in advance!

Best Answer - Chosen by Asker: Spray enamel will work best for outdoors - something like Tremclad. We re-painted an awning sign 10 years ago with Tremclad and it's only mildly faded in all that time, and none of it has peeled. I would suggest you wash the surface down first with a strong detergent.

I've also painted many other outdoor signs with Tremclad when I couldn't get One-Shot - that's a lead-based signwriter's paint which is the absolute best for use with a brush, but not available in aerosol.