Things You'll Need
Aluminum oxide sandpaper
Rusty metal primer
Rusty flashing is more than just an eyesore. The flashing in the valleys of your roof depends on a smooth surface for rain to wash debris off your roof. The flashing around vents, chimneys and skylights prevents rain from leaking in. Severely rusted flashing may warp, curl or even develop holes, exposing your home to the possibility of damaging leaks. Fortunately, if the rust is removed from the flashing before the structure is compromised, further deterioration can be prevented.
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Scrape away any large rust deposits using a stiff scraper or putty knife.
Scrub the rust using a stiff wire brush. Sand the area with coarse-grain aluminum oxide sandpaper. Alternate between the two, using the brush to loosen larger deposits and brush away dust and the sandpaper to scrub away the rust down to the bare metal.
Apply a phosphoric acid-based rust remover to any areas of rust you are unable to remove with the wire brush and sandpaper. Allow the rust remover to dry to a rough black finish. This black finish may either be sanded off or directly coated with primer.
Apply a rusty metal primer to the sanded area to keep the rust from recurring. Allow the primer to dry fully before topcoating with aluminum paint. Consult the primer label for drying time.
Topcoat the primed area with an aluminum paint. Aluminum paint is specially formulated to adhere to and mimic the appearance of the galvanized metals commonly used as flashing.
Check carefully at the joins where two sections of flashing meet. If the rust has spread under the join, it may be necessary to pry up the edges of the flashing in order to remove the rust. Leaving a rusty join may lead to further corrosion and possible leaks.
Never do any roof work without a properly secured safety rope. If you can't work safely on a roof, hire someone who can.
Priming or painting over rusty metal may not prevent the corrosion from continuing underneath the paint. Always remove or convert the rust before topcoating.