There is various options to get underseal off you can try petrol or something similar to remove it and i need be wire wheels etc. I have just done the underneath of my Rs with it only up on axle stands a pain in the hoop
I took it right back to the factory primer , then treated any surface rust then prepped the metal , sprayed 3M on the underside , then several coats of hammerite and for good measure waxyoil
Well worth it in the end for piece of mind etc