I just sealed bathroom sky light and fantastic fan and 2 other vents . Used dicor 501LSW (self leveling) great product. I, like Wildebill308 have used much silicone on many aircraft from SR71 to C130. Silicone is great for a lot of applications for what your doing the dicor is better. Lays down very nicely, holds better, looks better, is made for this job. Agree with Wildebill308 clean it very very well. You can use scotch bright and mineral spirits as per the directions. Also watch you tube videos. I also just laid 80 ft. Of 501 dicor on both sides of 40ft. coach, edges of roof to coach seal (fiberglass roof). Turned out good. The you tube videos will convince you.