You can find phosphoric acid in the masonry section of places like Home
Depot. It's used for etching concrete.
It has the advantage of attacking rust and not the base metal. It also
leaves a phosphate film on the material. Hydrochloric is more aggressive
and the surface tends to rust as soon as it dries out.
Not sure about MetalPrep for steel, but for cleaning/etching aluminum
before painting -
"alumiprep" is available at autobody supply houses for about $60 a
At my local plumbing supply I pickup a gallon of resin cleaner for my
water softener for about $20. Exactly the same composition -
phosphoric acid and a surfactant. Even the same percentages. (Rez
Care) (same as Pur Gard) (RezCare is about $17 on Amazpn)
I use it on steel too - but it doesn't contain any zinc so I get iron
phosphate but no zinc phosphate coating.
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