It will be necessary to sand the existing paint down to smooth. If after that the color remains undamaged, then a clear coat could be applied to provide gloss and final smoothness. If the color is damaged (burn through with the sanding, color shifts etc) then another coat of color will have to be applied.

It is harder to avoid orange peel with rattlecans than with proper painting equipment because you don't have much control over pressure and paint thinner. With proper equipment you can also add a ******er into the paint mix. Longer drying times make for less orange peel. Basically, orange peel is caused by paint not having enough time to flow over the surface.

Ultimately, with spray cans you have to get brave. The final coat or coats must be applied wet enough to have time to flow smooth. This will always be near the limit that produces runs. Practice, practice, practice.

On a bike frame, good practice is to paint the tubing junctions completely first because peel will not be as noticable in the joints. After all the thin dust coats, the main tubes can be painted in more or less complete sweeps. Start on the bottom surface of any given tube and work your way around to finish at the bottom again. The idea here is to have the final bit of overspray on the bottom as this again is one of the highest points of risk for orange peel.

Your going to have to provide pics as we are all eager to see the results.