Custom Paint - Workshop specials.

Best practice is to go fully raw and start fresh.

Good paint needs a mechanical bond and a chemical bond. Mechanical bonds are achieved through the key or abrasions in the substrate... chemical bonds are achieved through good product choice where the solvents chemically unite the layers.

If you paint onto the factory finish you'll only have a mechanical bond because there are no active solvents left in the paint on the frame. If that's okay for you, rub the bike down until it is all matte in appearance with no shine.... shiny means smooth and smooth means no key and no key means no mechanical bond. 500 grit is good. Red scotch will get into the gaps. That should be enough to help the paint adhere and is a good place to start.

Mechanical bonds alone aren't the most robust finishes. Painting over the factory finish might mean that you when you chip and scratch your finish you'll reveal the original paint... this will highlight that the weak link in your chain is the bond at that stage.

It works but it's not the best way to do it.

Starchem Synstrip is is a good product to get you back to raw.
One bottle might do four bikes if you're sparing. Maybe go splits on it with someone else in a similar situation. People often need it.