Late to the party, but I would definitely NOT do it!!! If i am reading your email right you are going through the expense and trouble to get the underside completely stripped and repainted - and you do not intend to use the car in the snow or bad weather? If that is the case why would you want to ruin a nice clean and painted underside by sparying goop all over it? If you are not going to run it in bad weather the underside is not going to see the elements that will eat it up and if you keep it clean and wipe it down once or twice a year the paint underneath will last and protect the metal just as on the exterior.