FOCUS ON RUST
QI noticed heavy surface rust on the suspension and subframes of my Ford Focus. I removed as much as possible with wire wheels in a minigrinder, then treated with a rust inhibitor. I applied two coats of red oxide primer, then two of a good quality primer, finished off with a coat of black. Now I see it's beginning to fester again. Why? And what would you have done?
Roy Bailey, via email
Matt says
ASuspension and other underbody parts are often irregularly shaped and inadequately-painted. As they're not visible, they're not usually subject to the stringent anti-corrosion measures applied to the bodywork. They lead a hard life, frequently wet and salty for weeks on end. The easiest practical thing to do is to apply a good quality underbody rustproofing wax during dry weather, having cleaned as best you can with wire brushes and scrapers. During a restoration, you'd remove and sandblast them. This eradicates rust from even the deepest crevices and gives a decent surface to take a paint finish, such as zinc or etch primer followed by epoxybased chassis paint. Unfortunately, trying to achieve a long-lasting painted finish without this back-to-zero approach sometimes gives disappointing results.
Quality underbody wax can be applied directly to a rusted surface, having removed as much loose or crusted rust and loose paint as possible.
It penetrates corrosion, displacing moisture and forming a waterrepellent barrier.
It's thus a single-stage operation once everything's been brushed and scraped. Painting is usually a multi-stage process and will only repel water as long as it doesn't get chipped itself. Wax coatings are to some extent selfhealing.