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QI’m restoring a Volkswagen van. I want the underside to be protected well enough to withstand everyday use – but I’m not going to be parking it at shows with mirrors under it. I’ve concluded that I don’t have the time or inclination to take all the old under-body coatings off, let alone take it to bare metal. Of course, there’ll have to be bare metal where I weld in repairs. What are your recommendations?
Tim Fisher, via email
AIf the old coatings are tenaciously attached to sound metal, they’re clearly doing their job well. We’d be inclined to leave them alone. Start by removing any loose clods or flakes. Then strip back any areas that justify suspicion – panel joints and seams, edges and folds, holes and orifices, and any places you suspect corrosion might be creeping under the coating.
Give your new repair sections a coat of zinc primer, seam-seal the edges, then apply a further coat of primer when the sealer’s hard. Degrease the metal and ‘key’ it with abrasive paper first. Finish by brushing two coats of enamel paint over the top of that. Some ‘anticorrosive’ enamel primers are suitable for use as a topcoat (it’ll say on the tin) and posh twopart epoxy underbody paints are available that can be applied without primer. Either alternative is fine.
Clean any areas of surface corrosion that you find with a twistknot wire brush in an angle grinder, followed, if possible, by a polycarbide disc. Use a wire brush bit in an electric drill and/or a manual wire brush in inaccessible areas. Ideally, then apply a chemical rust remover. Clean well and paint as before.
Finish by spraying the whole underside with a good quality rustproofing wax. This will need doing more often in areas exposed to road spray – annually, if possible –
but general underbody treatment can be left until the previous coating shows signs of flaking. Treat box-sections with cavity wax and retreat every three to five years.
It’s certainly not a necessity to take the whole underside back to spotless bare-metalled condition. You just need to treat any active rust, then monitor it and re-treat as necessary. The march of corrosion is thus slowed to a snail’s pace and any repairs are usually quite manageable.