Ayup, Phosphoric acid with zink dissolved. Probably 5-8% Phosphoric.
At least the Krauts provide an honest instruction sheet.
. Let FERTAN RUST CONVERTER work for at least 24 hours when used in temperatures of
55–95°F (12–35°C). Surface will turn black creating a protective zinc layer.
• In dry climates (i.e. direct sunlight over 95°F (35°C) or humidity under 50%),
spray light water mist after 1-2 hours to moisten treated area.
• In cold temperatures of 41–54°F (5–12°C), let work for at least 48 hours.
• Do not use under 41°F (5°C)
Key to acid conversion is thorough and complete wetting of the rust down to the microcraters into the good steel.
You don't do that you will find rust under the Iron Phosphate layer.
They probably charge out the eyeballs for it, and people will buy it because it's the New MIRACLE.
Actually it's a good process, done right no primer is needed, because the Iron Phosphate layer is bonded tot he steel, and enamel sticks well to Iron Phosphate.
And then there is the off label use of marine temporary repair polymeric coating called POR-15. Good slop for 6 months to get back to port so proper repair can be made, fantastic for seperating suckers from money.
If you want any of it in bulk it's available from the blender fairly cheap, but unless you're picking the barrel up at the loading dock freight and HazMat charges will kill you.
The molasses family of products aren't converters, they chelate rust away by stealing 1 Oxygen atom from the rust molecule. Work well IF you have a big tank and a bag of cow molasses from the feed store.
And lets not neglect mentioning the electro plating of rust off a desired object onto a collector. Not much different from good old replating of car bumpers we did in the 70s.
It's all about doing the process correctly.