So, I mixed silica fume with vinegar and spread over very rusty piece of metal:
I left piece of metal in vertical position outside. It is very warm and dry outside right now. Here is the next day:
It did affect the heavy rust, but it dried out to quickly... I decided to use the same mix, but covered it with plastic poly. Here is what happened in the morning:
This is after washing it off with warm water and plastic brush (no heavy scrubbing)
This is not bad at all for such a heavy rust, which I didn't even touch.
I think if I mix silica fume with vinegar to make it sprayable consistency I will be able to spray frame over every few hours and It will work great.
The other thing I am going to test is oxalic acid with silica fume and also phosphoric acid with silica fume.
Phosphoric acid is a rust inhibitor and I will use it before protecting steel anyways.