You probably want the tar, fiber-impregnated roofing seal compound called "Kool Patch" here in the states.

I had some serious rust damage to the driver's side floorpan of my 67 wagon. There was some less-serious damage to my passenger side as well. I used quite a lot of POR15 to seal up what was left, and I used the POR fiberglass floopan kit to rebuild the missing parts. Then.. I installed several Q-pad sound-deadening material and covered the whole thing with a layer of the roofing tar material. It is thick and is put on with a plastic putty knife.

BUT I do not recommend the plastic. I used it and it got everything safe to touch again, but it was amazingly slick and I could not get my newly cut replacement carpet to sit still on top of the stuff. With a good coat of Kool Patch you won't need to worry about water anyway, so I do not think there is good reason to use the plastic.