oil guard rustproofing is a thin viscosity oil that is sprayed on a car to prevent corrosion eg. undercarriage,frame,inside doors etc. cost is about $75.
Not needed. The oil collects dirt that traps moisture and plugs drain holes. Modern cars have treated metal and do not need this "protection".
I know a guy who told me his 8 year old van was almost rust free because of oil guard. He only had a little rust on the bottom of the doors and the bottom of his rear hatch. All the points were the drain plugs were blocked. If he did not use the oil guard it would be totally rust free.
Part of regular car care should be cleaning out the drain holes in your doors and truck etc. I usually check this when washing and waxing the car.
I've used similar rust prevention systems for 25 + years, and have not had any doors rusting out . and couple of vehicles have gone 275,000-300,000 km in winters in the Great White North's, sand, salt, chloride