Q: We have a 30-year-old galvanised-steel door that’s suddenly not holding paint. I used oil primer and numerous topcoats, but nothing helps.
A: The problem is saponification. The primer and paint you applied 30 years ago weathered away and let in moisture, which formed zinc oxide on the door’s surface. That oxide combines with oils that leach out of any new layers of paint you apply to create a crude form of soap. When you add more paint to it, the soap attacks the paint film and loosens it. Plus, when the Sun beats down on the damp door, any moisture under the paint turns into vapour and forces its way through the topcoat.
The solution is to strip as much of the paint as possible. Sand smooth any rough edges where the paint blistered and was scraped or brushed off. Next, apply a “prep and etch” product, which is a diluted form of phosphoric acid. You brush it on, let it set for 15 to 30 minutes, rinse thoroughly, and then let the door dry. As soon as possible after that, prime the door with a high-performance 100 per cent acrylic primer formulated for surfaces that might have an adhesion problem, such as vinyls, plastics and galvanised metals. Follow that with two coats of exterior acrylic topcoat.