How to Paint Rusted Metal
Remove loose rust
If you are working with a flat surface you can scrape away large sections of rust with a putty knife. If your metal isn’t flat you can use a wire brush. Be gentle, pushing too hard can damage the metal. After this stage, you should tap the metal with a hammer to make sure it holds together. This can save you time painting metal that might not be structurally stable.
Take some mid grit sandpaper to the surface of the metal to remove more rust. As you smooth the surface out be sure to clean off the metal and the paper frequently. If possible sand beyond the rusted area to blend this out.
Use a vacuum or shop vac to suck up the last of rust and the dust on the metal, if possible use a brush attachment. Then use a soft cloth with a degreaser to clean up any remaining rust, the goal is to remove as much as possible. You can even make your degreaser by combining one gallon of water with four tablespoons of liquid dish soap.
Even once you’ve cleaned the loose rust off you may still have serious surface rust. The good news is that you can cover this up, but not with paint alone. Paint will often either not stick to rust or will stain and discolor the coat. To solve this you can use a primer made specifically for this. With either water or oil-based rust conversion metal primer, you can chemically convert the rust to a flat black non-rustable surface. If you only have light rust or were able to get all the rust off you will want to use a standard rust preventative metal primer.
You’re finally ready to paint! If you did use a rust converter you should consult the instructions as some require an oil-based topcoat. Due to the black color of most rust converters, you may also need to do several coats, up to four.