In the days before car manufacturers applied much - if any - corrosion protection to their cars on the production line and professionally applied Dinitrol and Ziebart rustproofing treatments were very popular, I used to use the DIY version - Waxoyl. I’d liberally coat the underside, under the wheel arches and wings and inject the internal cavities of any new or nearly new car I bought with Waxoyl. The strong solvent smell was pretty unpleasant for a few days after application but it seemed to do a pretty decent job of stopping corrosion. By coating the underbody suspension parts too, there was never an issue with seized or rusted bolts or fixings if any underbody parts ever needed removing / replacing. I’d usually apply a top up application after a couple of years just for good measure.
Was this DIY rustproofing effective? Well, I saw one of my previously owned cars that I’d given the Waxoyl treatment to many years after I’d sold it. The bodywork still looked in great condition, with none of the usual front wing, wheel arch lip and sill corrosion that was very common and typical of cars of a similar age. So I’d say yes, it was effective but as with most DIY corrosion protection products, how effective they are will depend on how well and thoroughly they’re applied.
