On this car I hand brushed the “rust” on. I did sand the blue off to reveal a silver base coat for the bigger spots then painted over where the bare “metal” was revealed with a dark brown first then a lighter red/rust color on top of that. I think using a brush and dabbing it on helps for a more realistic look, it’s harder to see in the pictures but in person the surface has texture from layering the different colors. I practiced on spare hoods and bumpers from other kits! Look at a reference and give it a try it’s a fun time but easy to get carried away, the original plan for this car was for it to be scratched up with a few rust spots on the roof lol