Quote:
Originally Posted by deuce_roadster
Rust is iron oxide, IE oxidation of iron. Actually only O2 is needed. There is more free oxygen in air then water. However as soon as ferrous metal is exposed to air, it starts oxidizing due to mainly the O2 but there is H20 there also, that is why you need to "etch" it to stop the rust that has begun that you cannot see, but also seals the metal from the air with a thin coat of iron sulfate.
After I get anything sandblasted like a frame, I DA sand it to remove the sharp points left by the blasting material that removes the rust, immediately etch then shoot a catalyzed sealer (available now thankfully) on the parts for later primers and topcoats. Haven't had any come backs of rust yet and I have been doing this since the 1970s before the catalyzed paints were available. The etch is the key. I pay attention to the relative humidity as much as I do the temps before painting. My 2 cents.
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Etching primers are great for metal that has been stripped chemically. The etching primer has a light acid and zinc to "etch" or enhance adhesion by making a chemical bond to the steel. Modern epoxy primers are such great product for locking out moisture. Etching primers must be sealed by another primer, especially if it would ever be stored outside. The etching primer will put a hold on light flash rust from humidity. Still, a wet pc of metal needs to be dry regardless of primers and topcoats. I have found that an etching primer isnt necessary on sand blasted metal. And, everyone is going to have their preference and what works for them now and in the past. Most people have little experience painting raw metal and getting a beautiful topcoat. I just hope this helps someone not paint over moisture they dont believe exist on their raw steel parts. I always, on raw metal use epoxy, then while its still wet (1 hour dry time) apply 2 coats of Y2K, then after cutting dry Y2K, topcoat with paint of choice. This creates a fantastic wet on wet bond that is extremely durable and locks out moisture.