Fan installation

I have had the foam gaskets leak after a couple of years. They compress and lose their contact. Clean the old stuff off. Use a paint scraper and a bit of lacquer thinner to clean it spotless. Fill the holes with a dab of Dicor self leveling sealant and then a thin layer so it compresses well. Then be sure to bead the edges and cover the screw holes. No more problems after that. I do not like the butyl tape. Same issues with it compressing.

Neither. When you remove your existing fans there will be a lot of self leveling sealant left behind. You can remove some, but not all leaving an uneven surface which will be tough to seal with a gasket. The best way to install the new ones is to use the same sealant the factory did. Dicor self leveling sealant.

butyl for me.