Carb cleaner or brake cleaner are great to break RTV. If those don't do the trick, use a poking tool to break the seal then run the smallest screwdriver you can find across until you can break it by hand. Don't apply pressure at an angle with a tool, phenolic and aluminum don't like that.

Along with the thin lines above, you can run a ring around the outside for added insurance against leaks. Thats how I did it (without a gasket) and never ran into a problem after that.