Paint on Galvanised Sheet

Five years ago, carports were constructed with galvanized steel posts and beams, which were then painted after assembly. Details of the painting such as surface preparation or type of paint are not available.

Recently, the paint began to blister and flake in localized places, over roughly 10% of the area. After mechanical cleaning to remove loose paint (roughly 30% of the painted area was removed) in preparation for repainting, white deposits on the galvanized surface beneath the paint were observed. Also, some of the paint that was not removed during mechanical cleaning could be removed with adhesive tape. Thus, it is suspected that the surface of the galvanized was not properly prepared prior to painting, or that an inappropriate paint such as a alkyd was applied, or both.

There seems little doubt that repainting without removal of the remaining paint will lead to further blistering and flaking in a short time.

Ideally, one would want to remove all of the old paint and white powder from the surface of the galvanized without removing the galvanized coating itself. Is this realistic? What would be the best way to do it? Water jetting? Blast cleaning? Other?

 Low Pressure Water washing is up to 5000psi in NACE terms (Call it water blasting) You would also want a turbo head that produces a cutting action. This should get most of the paint off.

You could just sand the rest of the coating that is left. If you see that the coating is lumpy in places it can be a give away that white rust is under there unless it lumps from the galvanizing process.

Alternatively: If you could get a contractor with High Pressure Water Washing equipment (NACE terms -again -Up to 10000psi ) and strip the whole lot off with blowing the roof off the carports.

I would say that would be the quickest and best investment. There are probably contractors on this forum that could help you.

Paint companies generally don't