You can get rattle cans In silver or black at most engineering supplies, maybe Halford? Don't know the brand name and its in the truck pretty much every day
used some recently, wasn't very impressed with the screwfix stuff (I bought a load when it was on offer before)
I was touching up a van and used it to cover bare metal while I worked on the next bit... rust was cleaned off with a flap disk till metal was shiny... I'm not shy with the flap disk, took all rust off and cleaned an extra 20mm+ around the areas... guess it wasn't perfect because rust came back through the screwfix stuff after about 2 days
had a can of that ZG90 stuff lying around, so tried that... it was worse, didn't cover well at all and rust sports were through just as quick
I was shocked how quick rust came back through, thought any old zink paint would do for a few days on shiny metal
I *think* I was recommended it on here... bought it years ago and hadn't used it, found it at the back of a shelf and gave it a try... it was a hundred times better than the other stuff... better spray nozzle so paint came out a bit like out of a spray gun instead of an aerosol (a flat line) covered much much better... and no rust spots at all after a week... (van was re-sprayed after that so no idea how long it would last)
it's more expensive... but it's well worth the extra cost
just ordered myself another 4 cans now this thread reminded me!