What is the point of iron in DIY parking solution?

[HDS]

I was testing the common DIY recipe that you can find on the internet, the one that requires:
-Water
-Phosphoric acid
-Manganese dioxide
-Iron fillings / steel wool

I noticed the test results I got changed the properties of the surface, namely it smoothed it out. I got an AR barrel with a real park job that I compared the sand blasted part to before, aside from color they where identical in texture. Afterwards not so.

The process took about 20 minutes before it looked sufficiently dark as well. After this I was reading up on the parking process and on this page someone claimed the common DIY recipe floating around was a sham and what you really got out of that was an iron phosphate coating.

http://www.natscience.com/Uwe/Forum.aspx/chem/5533/Manganese-oxide-insoluble-in-acid

Then I read this wiki article here:
http://en.wikipedia.org/wiki/Phosphating

And it says that iron in the solution is not desired when trying to get a manganese phosphate coating. So this makes me wonder if that guy has a point.


So I am wondering that maybe I should skip the steel wool and any iron all together, maybe I should just let the acid and manganese dioxide powder dissolve first and then use only that and distilled water?

[Storydude]

without iron to season the Park mix, you'll be stripping iron ions from your work and not the bath.

when you want to strip the iron ions from the sacrifical Steelwool. ;)

[yarro]

If you don't saturate the solution with iron, the solution will take up iron from your part and not not deposit the maganese phosphate back in it's place. In layman's terms, parking works because at that temp, the solution would rather have iron than maganese so it swaps for it since the only way it can take more into solution when saturated is to get rid of maganese. The reason the reaction stops is at some point the maganese phospate coating cuts off access to the base metal or if your solution was depleted.

-yarro