sealing hard to get at areas

So I'm wondering what you do to seal metal from rust in places such as the back side of a tail light filler where its very hard to paint.

Ive always thought it helped to spray a weld thru primer on the backsides of any metal before sealing it all up to prevent rust from forming. 

---------------------------------------------Originally posted by LowLyfe83Ive always thought it helped to spray a weld thru primer on the backsides of any metal before sealing it all up to prevent rust from forming. ---------------------------------------------X2

3 m makes a material called rust fighter that comes in a can and a special paint sprayer that works pretty well . I use it in quarter panels and other hard to get areas