It depends on what type of finish you want,
Stone Guard is a water base spray product which is only designed to
give a ripple effect as you would see on later model cars,
Proofcote sound deadner spray in my opinion (I would Use)is more
like the manufactures finish underneath and unless you are
constantly driving on dirt roads will be easy to clean with a
hose,when you wash the top side of the car,you do the same under IF
you are driving it as a every day car,
Then there is the painted show finish BUT that will not stop road
noise and body hollow noise,
If you have gone to the effort of bare metal underneath in
preparation, a rust prevention paint is recomended.
Jim