4200

I'm installing new flanged ball valves on my E27. After just cutting and epoxy sealing 3 new backing blocks, I'm curious what I should use to adhere them to the fiberglass inside of the hull. I've read 4200 and 5200 (3M) are popular, and that 5200 is more permanent, and 4200 is good, but...

I need to remove a transducer installed in the hull this summer bedded with 4200. Suggestions concerning method to dig out the 4200 and any solvents to remove the 4200. Is hair dryer or soldering iron heat useful?