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Re: Re-visit of cleaning metal prior to painting
It has been sometime since I have worked with pre-zinc phosphated galvanize, but
it seems to me that it has either mill oil (a simple rust and oxidation (R&O)
treated mineral oil) or a stearate soap (for drawing and forming) on it. The
mill oil is easy cleaned with solvent or in a warm, medium-alkalinity, aqueous
cleaner. For aqueous, an immersion tank followed by a rinse would be better
than hand wiping. A more expensive appproach is an aqueous spray machine, but
they would need high production justify this investment. Stearate is harder to
deal with. It is a "soap" and in aqueous cleaning, needs temperature and
alkalinity to remove. These soaps are tricky if acid cleaners are used to clean
them. The acidity causes the soap to revert to the fatty stearic acid and
becomes difficult to remove.
The paint should be tested too. Thoughly solvent clean several boxes prior
to painting with each paint. If paint failure occurs, the problem is paint
related, although cleaning may still contribute the failure. Cutting up 4x8
inch panels would be a good approach to testing many combinations of
cleaning techniques and different paints.
Finding a direct, non VOC, non-chlorinated, non-HAP, solvent to replace the
present blend may be difficult. Acetone is not a VOC and may work, but be
advised that acetone has a very low flash point and is very flammable and should
be used with extreme caution.