udersealing and replacing sills

[coll_gt4]

just wonderd if anyone had any good tips on undersealing.The car is solid underneath but is getting stripped down to the shell for mechanical repairs.So thought i might aswell go the whole hog and underseal the whole thing as the car is 16 years old.Wwhats the best stuff to use,whats the best way to clean and prep the surface.

also one of the sills has been welded want to replace this my self.i have a mig welder and all the tools.But never replased a sill before.

any advice would be gr8
thanks in advance

[sb-1]

I would recommend using Waxoil or similar,this will allow the car to 'breath' underseal will just keep any moisture in.