Muriatic acid should do the trick. But by the time you take all safety precautions, and then neutralize the acid, clean the part, and dry it, you could have just used a coarse wire cup-wheel or silicon-carbide strip-wheel to knock off the paint/rust. Your call. I'm building a MIG cart right now out of scrap steel as well, but mine only had rust on it, no paint. I hate having to re-rust all the steel (six 10ft long sections of 1-1/2" 14gs square tubing), but I got it for $0.30/lb back before the pandemic. I ended up hitting all of them with the coarse wire cup wheel and nothing else since I'm stick welding it anyways.
