Pneumatic gasket scraper from Snap on tools. Part # pgs 1
Mineral spirits will soften about any kind of rubber. I use it to remove heavy coatings of RTV. Finding a way to soak it can be challenging though.
I have always used an acetylene / oxygen torch on the back of the board. It didn't take long to acquire a feel for how long to hold the torch in one place. Using a stiff putty knife on the front, the rubber peels off easily.
Many years ago I worked with a chap here in NZ who had a 200 gallon tank that was filled with a strong degreasing agent.
If you put a running board in and left it overnight the rubber was gone in the morning.
No affect on the steel.