I have a quart of "OSPHO" liquid that claims to turn rust into an inert substance, turning it black. It also states that you can paint over it. Has anyone used this product and what were the results. I am using it on the rear end housing and dust shields prior to painting.
Any feedback would be appreciated.

I have had some and used it alot, but on some projects that did not need a very high quality finish. It does what it is claimed to do, and can be painted over. Seems to work good on rougher body parts, truck beds, etc. It must be painted over, as it will not stand up to any weather or time, imho.

The rust is still there, the out layer of the rust granule is just "converted". Just wire brush the housing until it is as clean as you can get it then use the converter.

I think this is phosporic acid, sold at HD for about $8 / qt in a light green liquid. I've used it over 10 years now with no problems.
start by getting all the rust off you can, apply a coat, let fully dry, if color is dark gray, your fine, re-apply as needed until you get a uniform dark gray.

I use scotch pads (red) and scuff the whole thing (be aggressive), removing all the dusty top coat, clean and paint.