IMO its your car do it the way you want. The correct way is the cosmoline that resto rick sells. I just did a set of control arms for a 69 bee that was blasted. I primed them and sprayed them with a cast blast paint from dulicolor then taped off the line and sprayed the cosmoline from resto rick. It looks the look and they wont rust where not coated with his stuff. I also have just blasted the chassis parts and primed them with epoxy primer and painted them with a semi gloss black. All depends on what ya want to do with the car. I have one customer that painted the entire underbody of his car as nice as the top side. He wont drive that car. The car I am doing for him now we undercoated with resto rick's undercoating so he can drive and enjoy it. I have other cars that I have restored that we sprayed bedliner on the underside after blasting the bottom. Gives a heck of a protection for rocks and a great sound deadener too.