Originally Posted by
Kontact
Phosphate coatings are best thought of like hard anodizing - it uses the base metal and introduced chemicals to deposit a layer that both a coating and a conversion.
It isn't a coating like paint is, and won't react to paint stripper. There are chemicals that will strip phosphate, just like there are chemicals that will strip off anodizing. Or chrome.
The porous surface of phosphate isn't just good for absorbing oil, it hangs onto paint really well and is used that way for military weapons.
It is too bad that steel bikes don't get this kind of treatment normally, but chromoly is somewhat rust resistant. The reason cars no longer rust so badly is that they now galvanize the bodies before painting (zinc plating).
U'betcha. Premium auto chassis parts started getting painted real well back in the '90s, perhaps earlier. After welding, alkaline bath, then phosphoric acid pickle, then chrome sealer if I recall (not really visible, but it improves the rust resistance), waterborne enamel dip, then baked on. Tremendous salt-spray test resistance. Some customers spec'ed parts of the welded assembly to be galvanized, but the zinc would have been dissolved by the acid, so test parts were made with galvy but no acid pickle before paint, and no galvy but full pickle; The latter performed better, and was cheaper part cost, and didn't fill up the pickle with dissolved zinc, and didn't give off toxic zinc vapor fumes during welding. Win all around.
I treat steel with Ospho before prime and paint, it's a mild phosporic acid treatment, not nearly as deep an etch as the acid pickle above, but can't hurt. It's also nice to give that aged gray patina to a carbon steel kitchen knife, making it look like it's been slicing acidic foods like beef and tomatoes and lemons for decades. Old Sabatier carbon knives typically have that look. Did this to a new Old Timer carbon steel knife:
