I used to use Skyco Ospho Surface Prep. It was originally made for marine applications. It does a good job converting the rust but if used make sure that a) you wear gloves and googles (it can burn skin or leave rashes, and it will burn the eyes, I had a drop or two hit my eyes from the mist and it burnt bad, took 20 minutes of flushing....) b) use in a well ventilated area and c) you rinse very well afterwards. It can be either sprayed on or brushed on. Price point was $30 + when I last bought it in 2015 (it was available at Home Depot, but ordered mine through Amazon for cheaper and free shipping. There are different variations of this, I used the green lettered one. I liked it for what I used to use it for and I have tried a lot of different products.
I use this for vehicle frames and various metal parts and after cleaning and paint it lasts for a very long time. It can be used on clean metal (which 90% of the frames are) as a metal prep/protect.
Draw backs: a) can be expensive depending upon where bought
b) fumes can be bad
c) length of time before done, takes at least 12 hours (that was
my average time) before drying I believe it calls for 12 to 24
d) exposure to skin/eyes etc.. can cause burning etc...
This is not a product advertisement or endorsement
just my experience. My wife uses Rustbuster and Evapo rust remover in her little projects and they do a good job too, plus less expensive and enviroment friendly.
Just wanted to throw my two cents in. I think the Skyco Ospho Surface Prep
would be good if you have large objects or a lot of them to do. The others I mentioned that my wife uses are good for smaller objects but if you need a lot they will cost more in the long run.
HH