IMHO, I think you are asking for more work than will be justified...you'd be better off starting with a white airframe and prepping/painting to your desired scheme.

Biggest challenge will be matching colors and blending in the seams without making the effort stand out...also you'll need to protect the existing finish from damage while you fill, sand, and prep the seam areas. More work than doing from scratch, and if you are just cleaning up the seams on an otherwise "stock" color scheme, few people will notice the difference.

re clearing - depends on the gun and finish type you are using....generally a good scuff over the existing finish with the fine (gray?) Scotchbrite pad to break the shine, then color sand the newly sprayed areas with a minimum of 600 grit wet to blend and smooth the oversprayed areas, then one light tack coat of well thinned clear, let flash off then follow by a wet coat of clear.

I've used PPG DCU 2002 and Omni MC260, 2-part gloss clears with a Sata minijet HVLP, tack coats are thinned 80% with fast reducer, final coat thinned 50% with fast reducer (if below 70 degrees) or slow if above, running 40 PSI at the gun. This is more reducer than the mfrs recommend but keeps the weight down.

good luck!

Barry