Gonecamping444,
I just looked at your profile and see you have 2007 Chev Silverado D/A. If you are planning on spraying your undercarriage on a 2007 model, it's already too late. I see you are from Michigan. Michigan has some pretty salty roads in the winter. If you attempt to spray now, you'll just lock-in the rust and it will still continue to grow. Then if you need some kind of repair, you'll just have to fight through all the spray-on stuff to remove bolts and nuts and such.
Spraying now on a 2007 (if this is what you are planning), it's too late for the vehicle.... especially the bottom of your doors. There is no way to stop the rust now.