Bilt hamber rust killer (on ebay) buy the gel and then seal it up with tiger seal. Just kill all the rust after sanding or lightly grinding the worst off (you can get sandpaper discs to go in grinders). Once the tiger seal is dry you can just paint over it with white smooth hammerite.

Remember that most sound deadening matting is very good at stopping vibrations but you then really need a "decoupling" layer on top to really stop the sound like the sound deadening you are removing so a combination of both is best. I think newton commercial do bespoke ready cut kits.

If the floor is getting wet then you need to cure it first though.