Join Date: Dec 2010
Location: Adelaide (Modbury)
The car I purchased was sitting in 'black primer' for the last 20 years . When it came time to get rid of what I thought was black primer I was wrong, it was magnetite.
I did send the car to have it lightly blasted to get the black layer off. It could be the magnetite was too hard for the aluminum oxide or garnet abrasive which just bounced off the magnetite film, then the etch primer over the top the magnetite well some one forgot to tell me the black layer did not come off !, they just put lots of grey etch primer which was also very hard as well. I had the car sitting inside garage for about 2 years. then took it to the panel shop. The magnetite layer was still there under the grey etch primer I just did not know it, so not only the etched primer would not come off neither did the black film (magnetite) double trouble.
I just had to work it out as quickly as possible overnight as my panel/paint guys were upset as it was taking too long to prepare!
The simple job became complicated and more costly. I remembered some one said to me it was dipped before. The only black film/coating possible on mild steel was to have it dipped or in caustic soda (Sodium hydroxide). I don't think caustic = acid, Caustic soda = alkaline. This was the same technique they used back in the old days when they did not have access to high quality harden steels for gun barrels. That is why early gun barrels are blackish /blue. In one chemical article they talk about 'magnetite films produced on the mild steel surface could prevent or minimize the corrosion on its surface' which in effect what happened to my car as it had no paint on it for 20 years. Only problem is magnetite has a slippery structure so some paints primer / filler will not stick to it so compromising the job. Unless you like the black magnetite look.
I don't know all the details, but logic tells me if you leave painted steel (car or parts) in caustic soda NAOH and it works faster than you expect thus giving more time for the steel to be exposed you will get a thicker film layer of Magnetite, making it harder for you to get it off latter. We had to use Zirconium 3M sanding discs which got us mostly there.
Last edited by Steve105; 05-27-2018 at 11:20 AM.