Washing the exterior of your home before a paint job may reveal nails that have popped out of the siding or rusting nail heads that have left streaks of rust on exterior walls. If so, use sandpaper or steel wool to clean the nail heads. On clapboard siding, use a nail set to recess the nail head about ⅛ inch below the surface of the wood. Dab on a coat of rust-inhibiting primer (unless the nail is aluminum or nonrusting galvanized steel), and let it dry. Then fill the nail hole with spackle or putty. When the filler is dry, give it a coat of primer. For flathead nails, which cannot be recessed, sand the heads until they're shiny, and coat with primer.