flush mounted led's

ive got my rearskin all welded on and about to start the bodywork. ive the got leds that you can install then do bodywork/paint right over.question is.. the clear lense that you sand down to bodylevel. do you use some kind of adhesive to attach it to the skin? i was told that you should..then from someone else that i only have to bolt it in and run some clear caulking over the inside to protect from moisture.. then just sand the lense down and do my bodywork.... anyone have any trouble with it ever eventually cracking about the lense? thats my main worry is ill get it all nice and done then itll start to crack after its painted.

anyone have a name of what kind of epoxy i should get that wont hurt the plastic. only kind ive seen dries a light yellowish color i cant find any clear...anyone?