I've used the closed cell on our vintage pop-up. I also ended up tossing the closed cell gaskets because they got water loaded, deteriorated, leaked,and sucked up dirt like a sponge. For the class C, I used the elastometric sealant in the caulking tube. It took two coats, one when attaching the clearence light - filled the wire hole then squeezed out a base to set light on and another coat once it completely dried as I ended up with a pin hole or two around the edges of the light base. We also used Butyl tape (also called mobile home tape) to seal the metal corner mouldings on. I then covered those edges with more of the Elastometic caulk stuff. So far we are leak free. Parts of our RV are so beat up (looks like someone backed into a post or tree) that just the butyl tape or caulking alone would not seal. As far as our clearance lights, they had leaked in the past (apparently based on the gobs of slilicone I scraped off) and I figured the elastometic stuff would seal out the water better while remaining flexible.