If you try to simply paint a over a bare metal sign, the finish will fail. To prevent eventual chipping, condition the sign by adding an etching base that will ready the metal for a new finish. Unless you're blessed with a steady hand, you'll likely need to use a stencil to generate professional-looking letters and numbers. Affix the stencil to the sign, using the proper adhesive, or paint may bleed underneath. Choose a paint that will last for a long time, or you may be forced to refinish the metal often.
Things You'll Need
Canvas drop cloth
Rust-inhibiting metal-etching spray primer
Two colors of oil-based spray enamel
Stencil spray adhesive
Professional painter's tape
Sand down any rusted portions of the metal sign using a coarse 50-grit sandpaper.
Wash the sign with a degreasing soap, using steel wool. Rinse the metal sign, using a water hose. Let the sign dry for two hours.
Position a drop cloth beneath the sign.
Coat the metal sign with rust-inhibiting etching spray primer. Prevent runs by maintaining 8 inches between the sign and spray can nozzle. Prevent sags by keeping the spray flow consistently moving without stopping on any portion of the sign too long. Let the sign dry for three hours.
Paint the sign as you primed it, using an oil spray enamel. Let the sign dry for three hours.
Add letters or numbers to the sign, using a stencil. Coat the back of the stencil with stencil spray adhesive.
Affix the stencil to the metal sign; tape the edges in place, using a low-tack tape. Apply the second color of enamel to the exposed area of the stencil. Let the sign dry for an hour before removing the stencil.
Be sure to use a low-tack tape or the initial coat of enamel may be damaged.
Use a stencil adhesive to prevent the enamel from running behind the stencil.