I worked in the auto body repair business all my life, and have lots of experience with this.
First, most people often confuse rustproofing, and undercoating. They are two completely different products.
Rustproofing, is a paraffin based product, for inner panels. It is not supposed to "dry".
Undercoating is an asphalt based product, designed for the bottom panels. A good quality undercoating, will eventually "dry", but remain pliable.
Vehicles generally have minimum levels of corrosion resistance inside, and underneath them. Though the use of galvanized sheet metal by most manufacturers, has helped greatly in reducing body rust, a properly performed rust proofing, and undercoating job, can in fact extend the life of most vehicles.
Having cut apart many hundreds of vehicles, I got to see the results of all this. And, the effect it had on the vehicles.
Some rust is the result of a manufacturing design, or process, and therefore cannot be prevented.
The use of a soft, pliable, foam filler, between inner and outer panels, by many manufacturers often causes rust through in certain climates. You cannot remove the foam, or stop the corrosion it causes, short of cutting the panel open. This is what causes the rust, around gas filler doors on many vehicles.
The problem with aftermarket rustproofing, lies in the the fact that most rustproofing companies, failed to properly train their employees, and seemingly had them under apply the products.
Their warranty's required annual return trips, (at an additional cost to the owner), to have the vehicle resprayed. During which time, additional material should have been applied to find spots missed previously. Most of the time, these were also seemingly done in a fraudulent manor.
Miss one of those annual return trips for a touch up, and the warranty was voided.
The result was too often, rustproofing was applied only where the holes were drilled, and plugs were. But, if you moved inside 6" past that, there would be nothing.
The drilled holes were never sealed properly, leaving capillary action to hold moisture around the plugs, and ironically, rust to actually form as a result of the rustproofing job.
In some cases, when a rustproofing company tried to do the job "right" they would apply so much material they would seal off drain holes. Again, actually causing rust.
Undercoating on the other hand, is more difficult to mess up. And, is clearly visible to the owner, upon inspection, so it is often done more properly.
Vehicles that are parked outside, in anything but dry climates, could benefit greatly from a good undercoating job.
If you elect to have the underside sprayed, tell your rustproofer, you will require them to put the vehicle on the lift, for your inspection, when you pick it up. Take a good flashlight, and look on top of the frame rails, and inside any openings.
On a used vehicle, it is important to have the underside clean, and of course dry, which is going to be your challenge. It doesn't have to be spotless. But, it does have to be free of mud, and dirt. And, any rust that is scaling, or flaking needs to be removed.
If I owned a vehicle, that has areas know to rot. Such as the tailgates, on one truck manufacturer in particular, I would either have those areas rustproofed, or rustproof them myself.
Body shop supply stores sell rustproofing in aerosol cans. It's the same stuff the rustproofers use, only thinner, so it will come out of a spray can. You can, with some minor disassembly, do a nice job rustproofing the bottom of the common rust areas, (which is all that really maters).
Just make sure you buy, rustproofing, and NOT undercoating for this job.
Aerosol rustproofing only comes in an amber color, and will cost you $15-$20 a can. Expect the counter person to show you a can that says "Undercoating" on it, and confidently tell you it's the same thing. :mur: Don't try to educate them, it's hopeless. Move to the next store, or order it from the internet.
Aerosol undercoating is of course, also available, for the underside. But, even the expensive stuff, is not as good as the professional products. It's more for looks, than protection.
Keep in mind these products have high odor levels for several months. I would wait until fall, if those odors offend you. Because, the hot weather accentuates the off gassing.
