Gazim,
without looking it sounds like the clear coat is getting a good dose of fumes/unburnt fuel circling back against the tailgate and is discoloring your clear coat.
I would guess rerouting your exhaust tips will be the only cure ...sanding an waxin is gonna leave ya with a dead paint really soon (CLEAR COAT GONE).
maybe flaring the tips more out to the side to catch the free flowing air as ya drive may be an option or double check your Carbs mix and motor timing to get a cleaner burn ?

Tim


1952 Chevy Shortbed

Photos in Photobucket

If It Aint Broke Fix It Till It Is!