It depends on the quality of the paint already on the bike. If good, then a light sand with 800 grit will do so the old surface provides a key for the new. Chips and scratches will need to be sanded back to the metal (and remove any corrosion), and primed with a metal primer. The primer then also will need spot sanding to blend it in with the original paint. Then spray away.
The following is a post I made on the subject in another BF forum:
Use a face mask... the spray drift can be unpleasant, let alone dangerous.
I've used paint remover and a scraper to get the original coats off. It's cleaner than sanding. On old bikes, I don't know if lead was widely used in the paints.
You may need three or four remover applications to get down to the undercoat through the clear and coloured finishes. This especially is useful around the lugs. Use a fine-grit sandpaper, preferably wet. Then get a tack-cloth from an auto paint shop, or use balls of masking tape to remove any dust on the metal before the first primer.
Pick days when it's not too hot, not too cold and not too humid to start the finish coats.
Use several coats of primer, lightly sanding back with fine paper, and repeat the wash and de-dusting process. Don't dwell with the spray, use feathering movements back and forwards. Start and finish each pass of the spray away from the frame. Places to watch for and are the most difficult are the inside the joins at seat/top, top/head and of course the bottom bracket. There is a temptation to just point and squirt, but then you'll get paint runs.
You will need four or five light coats of colour paint, and after each you will need to wipe down the frame to remove the overspray. Sanding in between with a very fine grit is advisable to (a) smooth down dust, insects or paint runs and (b) create a surface that the next paint layer will bond to.
By the third coat you should be getting real depth and shine to the colour. It may take a day or more to dry between each coat, and this is important if you are wet-and-dry sanding down (or even dry-sanding) between coats. The temptation is to think that each coat takes but minutes to apply, and to whack on some more.
Take your time.
Finish with three or four coats of clear for added protection. Apply any decals, preferably cut to shape, before the clear finish (laser-cut vinyl signage is really durable, but creates a relief feel if you get fancy with outlined letters and stuff -- talk to a signwriter about what's best).
Auto finishes aren't the best for bikes because they do tend to chip easily. Ironically, for the several muckabout frames that I have sprayed, I have used cheap enamels and they have stood up remarkably well with good finishes. As to colour choice, pick a shade or two *darker* than what you expect -- for some reason, bike frames tend to wash out colours.