The ospho bottle says it it converts iron oxide (rust) into iron phosphate, an inert hard substance that turns metal black. So if the rust is no longer rust, then why, when I sand the now black inert substance, it powders off as brown rust colored dust? Hmmmmm...
Rust: the way iron comes from, or returns, to the earth. Oxide: iron & oxygen (our sometimes friend ). Phosphate does convert iron to a new compound. I've always liked the black finish. Possible the rust was the earlier top coat.
Confusing subject. If you look at the bottle, or the website, it says you do not need to sand, or neutralize, just go ahead and paint. However, if you do a google search with "HAMB" included, you will find that most folks say the opposite. Take your pick! I sprayed it on too thick once, 3 days in a wood stove heated shop, and it never dried. Had to wipe it all down with a damp rag
I was using Ospho today on a surface rusty '29 truck that's been left outside. I was planning on brush painting the thing with Rustoleum just to keep it from coming back so fast since I don't plan on working on it for a while.
I built a '47 Ford pickup for one of my grandsons fifteen years ago. I used Ospho on rust. Still no sign whatsoever of rust.
I just pulled a dismantled Model A cab out of a non-climate-controlled storage unit, about a mile from the salty San Francisco Bay. It is bare metal, coated with Ospho. Zero rust in one-decade.
Ospho is phosphoric acid as the active ingredient. The iron phosphate that is on the surface is a good crystalline surface for paint adhesion. Iron phosphate by itself it is not a real good corrosion protection method. Better than bare steel, but not as good as paint for example. Iron phosphate plus paint is better than paint alone. Straight iron phosphate is grey color, fasteners that are phosphate coated have a drying oil put on after the phosphate which makes them dark grey or black, which acts like a sealing coat and improves corrosion protection. But paint over phosphate ( no oil) is better.
Buy some POR metal prep. I like the results better. Red scotchbrite pad to knock the loose stuff off, then follow the directions on the bottle.
Layman's Terms... Basically what these treatments do is "Blue-Brown-Blacken the steel much like rust blueing a gun barrel.
So I’m still wondering why when sanding black, it comes off as brown? Does the ospho only convert the very top surface, and the sanding cuts through to the concealed rust?
I've experienced the same thing. And I think you're right, the acid hasn't soaked in/converted all the way.
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