I haven't replied for a bit but the project is underway. Here's an update of where it's at.
Steve. Thanks for your response. Ken Carpenter said that the original sealant would have been Acryl-R, which is a self-levelling product. What we’re looking at bonding with new sealant is on the vertical surface at the outer edge of the slide, and must be after manufacture. It is still a bit flexible and cuts cleanly, reminiscent of Boat Life polysulfide. There seem to be some bubbles here and there in it. Over that someone else used white silicone, which adhered only on the horizontal surface beyond the area needing to be caulked. It looks like there were 2 applications of sealant over the original, with no cleaning done at all. We have some test pieces of the old sealant and are testing with Dicor non-sag sealant to see whether that will bond. While we get that organized, we have to do a lot of cleaning to figure out what is actually leaking as well as to get a good surface to bond to. This isn't such a fun project that we want to do it twice!
Ed. Thanks for the tool suggestion.