Makers beware!
Much like with common household chemicals used for cleaning, such as bleach and ammonia, improper use of these can produce e.g. chlorine gas, which while harmful is generally not lethal. Things get much more serious with brake cleaner, containing tetrachloroethylene. As explained in the video, getting brake cleaner on a rusty part to clean it and then exposing it to the intensive energies of the welding process suffices to create phosgene.
What’s wrong with using a wire brush or an angle grinder to remove the rust?
To be clear, brake clean isn’t removing any rust.
For welding, some kind of grinding is definitely occurring.
If the brake cleaner doesn’t remove any rust, why do they use it?
Welds need a super clean surface to start with, if there is any oil or rust it will form a layer within the weld where the metal won’t melt together properly. Brake cleaner doesn’t really leave much of a residue, and it is an excellent degreaser. Problem is when you empty a can onto a prepped weld surface surrounded by tons of rust and gunk, it’s the solvent that gets trapped in there and blasted with heat and intense UV from the weld arc that can form phosgene. It can also happen when people try to torch rusted hubs after trying to use a can to break rotors and the like loose, that’s one of the reasons they moved to non-chlorinated stuff most places.
To remove everything else like dirt and grease. I use it almost like an all purpose cleaner when working in the garage
Wire and rust embedded in your skin is not good. In the eye is worse - I know of people who were wearing both a face shield and full safety glasses who got grinding dust in their eyes.
I don’t have a good answer to the problem, nobody else likes any of their answers either from what I can tell.
The time and effort its takes evidently
Mechanical then chemical cleaning is pretty normal, at least around here.