Welding Tips and Tricks

This may be a silly question, but I'm not sure of the simplest answer.

I do a few welding projects here and there for my masonry company, usually made out of hot-rolled mild steel. The problem is this: these metal structures often sit outside on a construction site for months at a time, and although I've tried giving the a couple coats of rust paint, they get fairly rusty after a while.

I think the problem tends to be that the steel is hot-rolled, so covered in mill scale, and no paint sticks to it that well. The only solutions I've come up with are to either descale the steel (how, I'm not sure, I don't know how I'd "soak" a 10 foot long piece of square tube in acid, and then get it ready for painting), or to work with cold-rolled steel, although the cost isn't appealing.

I feel there must be a simpler solution to just keep these sorts of projects from rusting, especially since aesthetics aren't that important.

I use Rust-Oleum paint for a first coat. Lasts for a year or so.

sand blast and then paint, or something like THIS

Boeshield has 2 products, Rust Free and T-9.

Rust Free will remove rust and T-9 will prevent it.

Rust Free actually has a pleasant smell to it, and does a good job at removing rust.

T-9 less pleasant, but it does a good job at preventing it.

Unfortunately if you leave it outside I'm not sure it will endure, you may need to re-spray it with T-9 from time to time. At least that is my experience indoors, but it does a pretty good job at keeping rust at bay on stuff like Lathe chucks and other metalworking tooling. As a plus the plastic coating on electrical wiring is not effected by T-9 and it is highly recommended to spray on electrical of marine products such as boats which are notorious for rusting and corrosion. T-9 is also a penetrating lubricant. I highly recommend it as a replacement for what people use WD-40 on, I only use WD-40 as a cutting fluid on aluminum. Other than that, it's crap, unless you need to displace water which is was designed to do. It gums up and destroys mechanical parts, IMO.

There are various places you can buy it online, but Rust Free is harder to find.

I've had good success with this https://aquasteel.co.uk/rust-treatment/ on both new & second hand material - it goes on looking like milk & as it reacts it turns blue or black - once cure any normal paint will bond to it - stinks though while curing.