Maybe ya just missed this earlier Woody, but here it is again;
"Just one thing on the acid etch primers. Don’t get lulled into thinking that this is what they are formulated for, they are all too often misused. They are designed to be used on the new metal that has a galvanized coating not for bare steel. Just read any of the spec sheets from any of the paint manufactures."
When you use an metal prep to remove "light" rust you just can't leave the resadue on the metal, you need to wipe it down after it has done it's job. Your objective is to desolve the surface a few microns deep and leave clean bare steel.
Etching primer is used to give the galvanized surfaces tooth for the primer to grab on to. The bare zinc coated metal still needs to be clean before applying the etching primers. Forget the etching primers for what you are doing!
Another explanation of Metal Prep from Metal Web News, I quit after this;
“Another technique for removing rust is etching with Phosphoric Acid. Phosphoric Acid has a unique property of dissolving iron oxide quickly while etching iron very slowly. This means that you can leave metal in Phosphoric Acid for much longer than necessary with very little damage. The acid will attack bare metal slowly and will start the process of hydrogen embrittlement, so use the minimum etch time that removes all rust.
Another unique advantage of Phosphoric Acid is that it leaves a fine coating of iron phosphate behind. Iron phosphate prevents rust. However, the iron phosphate coating is not very thick and not durable. Some additional protection is still required.
Phosphoric Acid etch will leave a hard, bright metal finish. This is because it will etch the surface slightly, exposing new, bare metal. Often this is desirable. It leaves an attractive surface and a surface ready to paint. A common product which contains Phosphoric Acid is Naval Jelly. The soft drink Coca-Cola contains Phosphoric Acid, so Coke will etch rust. But Coke also contains carbonic acid and other nasty things. You're going to drink that stuff?
Auto body shops treat metal with acid metal wash, a solution of Phosphoric Acid and alcohol before painting. This removes waxes and oils, removes slight amounts of rust that form between sand blasting and painting, and leaves a thin protective coat of iron phosphate. One commercial solution for this is DuPont Quick-Prep. Sherwin Williams has a similar product called Metal Prep.”
Denny Graham
Sandwich, IL
