Wanna better more realistic method? Dig this....Paint whatever you want to look rusted FIRST with a THICK coat of red primer. Then, sprinkle some sea salt over the surface where you want that kinda heavy rusted look. then apply your finish coat. After it dries, gently brush over the area with a stiff paint brush, so that it will knock off SOME but not all of the sea salt. the missing pieces will reveal the primer and look like rust, while the remaining pieces covered with the finish coat will appear to be bubblin up. Works real nice.