Basics of Painting Rusted Metal
Can You Paint on the Rust Itself?
Whenever possible, it is always best to sand off the rust and expose only bare, unrusted metal for painting. But it is possible to paint directly on top of the rust, as long as the surface is stable. This paint should always be laid on top of a primer designed for metal.
Will the Rust Show Through the Paint?
Untreated reddish-brown rust can bleed through paint, especially lighter color paints. Rust conversion primer turns the rust black and its polymers seal the rust against bleeding. As long as the metal is properly primed, the rust should not show through the paint.
Can You Stabilize the Rust Before Painting?
Oxygen, water, and iron are required for oxidation—the process that creates rust. Using a rust conversion primer before painting halts oxidation. Tannins in the rust converter turn the rust black and stop the oxidation. Polymers in the converter seal the metal against oxygen and water.
When Is an Item Too Rusted to be Painted?
Generally, items that are deeply pitted or pinholed are too rusted for painting. Often, an item might look promising at first. Yet scraping and brushing take off layer after layer of rust, revealing no substantial metal underneath.
Safety Considerations
Painted items with rust may have been earlier painted with lead-based paint. Homes built before 1978 may have lead-based paint. Metal items popular with crafting and restoration such as patio chairs, tables, signs, and housewares may be painted in lead-based paint, too. Observe all safety techniques for working with lead-based paint.
What You'll Need
Equipment / Tools
- Wire brush
- Sandpaper
- Shop vacuum
- Putty knife or five-in-one tool
- Latex gloves
- Eye and breathing protection
- Hammer
Materials
- Degreaser or denatured alcohol
- Rust converter primer
- Water- or oil-based paint
Remove Loose Rust
Use the wire brush to slough away all of the loose rust and any paint. Begin lightly, removing only the rust layers that come off easily. Use a putty knife or a five-in-one tool to scrape away large sections of rust.
Once the large rusted sections are gone, tap the metal lightly with the hammer to assess whether the metal is strong enough to be painted. It is usually at this point that structurally unstable metal will collapse.
Sand the Rust
Sand the rusted metal to remove more of the rust and to smooth down the surface. Frequently clean off the surface and the sandpaper. Sand a few inches beyond the rusted area, as well.
Clean the Surface
Use the brush attachment on the shop vacuum to clean off the surface. Use a degreaser or denatured alcohol to clean off any grease or oil. It is difficult to get a rusted area perfectly clean, so the goal is to remove the majority of the flakes and dust.
Apply the Rust Converter
With the rusted area cleaned off and dry, spray the rust converter over the rusted area and a few inches beyond. Apply a thin coat and avoid drips. Many rust converters are clear when they are first applied, then turn black. Let the rust converter dry for at least 24 hours.
Paint the Metal
Apply the top color coat. Some rust converters may require only an oil-based top coat, not a water-based paint; consult the rust converter instructions. Because the rust converter is black, you may need to apply three or four coats of paint to reach the desired color.
Tips For Painting Rusted Metal
- After brushing and sanding the rusted metal, move to the priming stage fairly quickly to avoid the metal from rusting up again.
- Small pitted or corroded areas can be filled in and smoothed over with a fiberglass-based filler.
- If the area is large or particularly difficult, use a wire wheel attached to a hand grinder instead of brushing with the hand brush.