In order to make it stick properly, yes, you probably will need to sand it lightly with something like 600 grit wet paper. As for the shine, I'm not sure about clear coats for plastic, but for metal, clear coats come in gloss, semi-gloss, and flat. If the container doesn't say anything about gloss, then I would try it on a scrap piece of plastic to see what it will be like, before putting it on the tank cover. And about the UV rays, unless it specifically contains ingredients designed to stop the rays, then it probably will do nothing in that department.
2003 Ford F-350 V-10 Crew Cab 4WD Long Bed
2004 Jayco Designer Medallion 29 RLTS 5th wheel
Bill, Gayle, and Mindi (dachshund)
Retired at last !
Nothing is foolproof to a sufficiently talented fool.