Please elaborate, I am uninformed, and of the impression that you cant just reapply another clear coat.
Mmm kay, and this is speaking from experience:
-Using 1200 grit wet/dry sandpaper, wet sand lifting/faded/crumbling clear coat, being careful to ONLY get clear coat and not the base/color coat (on a BMW 325si I recently did, that took me a couple hours to do the whole car). The intent is to make the substrate nice and smooth/consistent, but not shiny. Cost: $4 for the paper
-using a 3M brillo green pad, scuff any other areas you wish to respray. Intent is to make the finish milky white/slightly rough, and to give new clear something to hang on to. Cost: $6 for a pack of the stuff
-using a grease/polish remover, wipe away any remaining wax, grease, grime, etc. Simple Green is good stuff and is cheap. I like acetone (nail polish remover), but I use it sparingly on a lint free rag.
-using painter's tape and plastic (not newspaper), mask off everything not being resprayed. This is critical. Cost: $4 for the whole roll of tape and $<another cost for plastic, depending what you're using. Split-open trash bags work well for small areas>. Masking off can take minutes, or hours, depending how much you do.
-respray clearcoat in nice even strokes, with a 50% overlap. DuPont sells the stuff in rattle cans ($38 per) but those costs vary up and down depending on manufacturer. I have a compressor and painting rig so I buy the clearcoat and activator in separate containers, and mix them as appropriate. Application time varies, but that BMW took 10 minutes to respray one coat in its entirety, much much more if I used rattlecans.
Note that if you're repairing only a portion of the car, it's best to do the entire body panel (the entire door, the entire hood, the entire trunk, etc). Blending clearcoat is a bitch, and applying it along natural body lines of the car is much much easier (that is, doing that whole body part down to a trim piece, where a pillar meets the body, etc).
After two or three medium coats, allow the stuff to cure. Cure time between coats is 10-20 minutes depending on manufacturer and environmentals, and this is critical to note before starting the job - too much too soon = runs, sags and drips. Final cure time should be a minimum of 24 hours, but again, that depends on temps and other environmentals. I recently painted a Hyundai that cured to the touch in 30 minutes and was rock solid in 12 hrs. That was a really high end clear though, and was solvent based.
Cited cure times are based on my sort of heated garage - I don't have a car oven.
I'd recommend not applying clear coat in direct sunlight. It'll harden too quickly on an already-heated surface and multiple coats will peel off one another.
For anyone looking to learn more, go to youtube and view the Eastwood videos on the subject, the ones from DuPont or the ones from 3M. There are a bunch of other professionals who put out their own series of related videos as well.
If interested, I can put up pics of the BMW and Hyundai I redid (I'm pretty happy with the result)