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Parkerizing (manganese phosphate) different types of metal

I'm parkerizing an ornamental piece of cast iron and hot rolled steel.

The formula I'm using (phosphoric acid, manganese, water) works great on the cast iron. I get a very durable finish.

On the hot rolled steel however the finish is much less durable. I can't rub it off with a finger, but an aggressive stroke with a wire brush leaves bare metal scratches.

I've tried increasing the temperature, using more acid, using more manganese, cleaning the steel better-- nothing seems to help. Is it simply that different metals "take" the parkerizing better?

Thanks alot

Sam Hardy
artist - Phoenix, Arizona
2006

Phosphated surfaces must be oil,wax or lacquer finished.Instead your solution you can use 20 gm tannic acid ⇦this on eBay or Amazon [affil links] /15 ml phosphoric acid/0,8 lit water solution/ 0,2 lit pure alcohol(simple and cheap do it yourself rust converter ⇦this on eBay or Amazon [affil links] ). Hope it helps and good luck!

Goran Budija
- Cerovski vrh Croatia
2006

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Thank you Goran

I've appreciated your responses on other threads too!

Sam Hardy
- Phoenix, Arizona
2006