On 87 Dolphin, I found 2 roof vents were leaking because they had all-plastic frames which had cracked in two or four places. I decided not to try sealing the cracks because on one frame the cracks were too long. They would probably flex a bit and the seal would not hold. So I bought new vents with metal frames. The question is how much of the old caulk has to be removed to get a reliable seal with the new caulk. I bought caulk tape to use. Scraping the old caulk will not get it all because the roof top is aluminum and embossed with a pattern similar to wood grain. I can use a plastic scraper to get down to the embossed ridge tops but all the valleys still have the old stuff in. I can't scrape too hard because the aluminum is too soft (and painted). also the old caulk is two kinds, a softer material which can be pulled and stretched is under the flange. A different material was put on later in a swath an inch wide or more over the flange and onto the roof. This stuff is very firm and hard to get up. I have tried using a metal putty knife with a thin blade to get under it but it cuts into the aluminum roofing where the high spots are, so I would rather stick with the plastic scraper. Is there anything else I can do, such as a solvent? Oops I didn't mention I also have 100% silicon caulk to apply after the new roof vents are screwed down. (And MaxAir hoods go on too)