Most undercoatings only work on new cars. The reason is the undercoating is applyed to a clean, newly painted surface. Even when new, the applicators wash and steam clean the underside of the car, to give the undercoater a clean surface to stick to.
POR15 and its counterparts create a chemical bond with the metal.
All the tar will do is cover the metal. If there is a small hole in the tar it will hold water and cause rust.
Most roofing tars don't "dry" compleatly. They are designed not to. They form a hard surface and are pliable underneath, so they can expand and contract with the shingles or as your house moves. Ever remove 20 yr old shingles? The tar they used around the flashings is usually still pliable.
GravelGuard/under coater is a sprayable tar designed to protect the sides and underside of your car from rocks. It is thin, but the reason it works is that it "moves" when struck by the rocks. It still needs paint over it to protect it. If the surface the GG/undercoater is put on isn't compleatly clean, rust will form & it will fall off. Might take a year or two, but it will happen.
Use a product for what it was intended.
Why paint your whole house, when only the neibours look at the front? Because you will know that it's done right
If your keeping the car, spend the extra $$, do it right. If your selling, spray/tar away.