I use 3M paint removal wheels "or equivalent" for heavy rust scale, super flexable nyalox nylon dico brushes, they come in 3 grits great for paint/rust to 1st stage burnishing/polishing soft metals brass alum ect great for carb stuff.. Wire drill brush wheel for threads, evaporust "of course" use soked paper towels plastic wrap & magnets for large items or I like rustolium gell rust disolver spray & for paint, the jug of klean strip paint remover snot used with a paint brush, heavy duty cleaning gloves a small metal brush & a large automotive drip sheet...
I have used phosphoric acid 75% to remove rust out of a gas tank of a Puch maxi moped. It worked above my expectations.
I had put 0,5 ltr in gas tank, for about 15 min. with some rusty steel nuts and shaked the tank once in a while pretty hard to let the nuts do their work. After 15 min drained the tank and got nuts out with magnet. Tank was clean as far as I could see. Rinsed it with water and dried it quikly (heat, compressed air). 2-K coating applied afterwards. In video below from around 30-35 sec.
I recently did a three wheeled tractor frame & fenders with muriatic acid be soaking terry cloth towels in the acid and draping them over the parts. I then covered them with a tarp for about 14-16 hours. Then I washed the parts with a baking soda & water solution to neutralize the acid. Once dry the parts looked like they had been sandblasted. Worked for me!
I have some phosphoric acid that I plan to spray on some angle iron and let sit overnight, as directed by the instructions on the jug. It's supposed to convert iron oxide (rust) into iron phosphate, which can be rinsed off with water. I think of this as a sprayable Naval Jelly. I have used Naval jelly on small stuff for decades, but I now have several larger projects that need to be cleaned up for paint.
He estimated $120 to blast it but he also suggested I have everything ready to paint it before I drop it off. I am still waiting for a few parts, plus, I want to see how well this phosphoric acid works before I decide how to proceed.
I got good results with the Clean Strip concrete and metal prep. I got a cheap one gallon pump sprayer and sprayed some plows and other implements they stay outside. Nothing seems to happen until it rains, then it all turns streaky white like spilled milk. At that point, the rust is stopped according to my Google research. My pressure washer blew the white crust off and left the metal dull grey. Those do not get painted, they should have been treated with Fluid Film after they were used. I also sprayed another protect that was painted back in the sixties. It has been in the woods for at least thirty years and has light surface rust and very little paint left. After I sprayed it, I pushed it into the garage for some major "repurposing". It was much easier to grind and weld on it after treatment. The rust is white chalky. Have not pressure washed that one, yet. My son and I have been making a three wheeled go kart from spare parts and scrap found behind the barn. Some painted, some rusty, some stainless. We are done tacking stuff onto it, so we removed the seat engine, handlebars cables , chains and jack shaft so I can put it on the bench and have a welding party on it before painting. I sprayed it, waited overnight and pressure washed it. I am amazed at how it looks. Will post pics of it thus afternoon. No rust, but most of the paint is still there.