4 ways to tackle rust | Good, better, best, ideal

By Per Schroeder
Mar 5, 2023 | DIY, rust, how-to | Posted in Shop Work , Restoration & Renovation | From the Aug. 2009 issue | Never miss an article

Photography by GRM staff

For those of us who tinker with cars, the common denominator isn’t steering wheels or pistons—it’s rust. We’ve all felt the same twinge of fear when it comes to this threat. No matter how old a car is—even if it’s fresh off the showroom floor—there’s rust lurking somewhere on it, waiting to spread. Fiberglass-bodied cars are no exception, as they have steel frames and fittings that will rust if allowed.

The best method for treating rusty steel is a much-debated subject. Some say that the only way to properly fix rust is to completely remove and replace the affected metal, while others believe that pitted but solid material can be treated with various paints and potions.

We’ve been working with rusty hulks for decades—easily since before the birth of this magazine—and all of that experience has taught us something: Each method of dealing with rust has its pros and cons. Here’s how four of the most common solutions compare.

Good: Rust-Converter Paints

Examples: Krylon Rust Converter, Mar-Hyde One-Step

This one is almost too appealing to pass up: A spray-on product simply converts the rust to an inert black coating. This coating can then be covered with a top coat of paint.

Many rust-related products sound too good to be true. Could a spray-on coating really convert rust to a thick primer? Turns out it can. While these rust converter paints are handy for preventing rust from spreading, using them as a base can result in a top coat that’s less than smooth.

Pros: These spray-type products come in easy-to-use rattle cans. They are inexpensive and can be sourced from just about any big-box store or auto parts dealer.
Cons: We’ve found that this type of coating isn’t as effective as removing the rust before spraying paint. Unless all of the rust is first removed, the treatment generally only delays its return. Rust converters will also typically leave a rough surface that can be difficult to paint.

Better: Rust Encapsulators

Examples: POR-15, Rust Bullet, Eastwood Rust Encapsulator, Zero-Rust

The concept of a rust-encapsulating paint also seems too good to be true. In reality, however, these rust treatments are capable of creating a great barrier that stops corrosion from spreading—given proper application, of course. We’ve found that encapsulators work well if you follow the instructions to the letter. Don’t expect the product to work on layers of flaking rust or adhere to a slick painted surface, as the rust encapsulator needs a rough yet solid foundation. Holes present a similar issue; sadly, we have yet to find a way to paint over air.

If a rusty surface is sealed off from oxygen with an encapsulating paint, the rust will not spread—in theory. These paints aren’t ideal for use on exterior sheet metal, but they work great on suspension parts, trunk floors and roll cages.

Pros: These coatings will produce a tough finish that seals the surface, yielding good impact and chemical resistance. They are great for thick-gauge steel such as suspension arms, subframes and roll cages. The products can also be brushed on, and they flow well enough that the brush strokes are rarely noticed.
Cons: Rust-encapsulating paints will often flake from smooth or glossy preexisting finishes. The color pallette is usually limited—typically only black, gray and silver are available. Plus, these coatings are hard to remove from hands, clothes and just about anything else. And since they’re a permanent solution, they’re very hard to remove once applied to steel.

The concept of correct surface prep is also subject to debate: Some products require surface conditioners, acid etches and primers, while others are designed to work as one-step processes.

Best: Three-Step Process

If the metal is only afflicted with a bit of surface rust, sometimes road cancer can be stopped in its tracks with the traditional three-step process: Clean off the rust scale with a wire brush, sandpaper or grinding wheel; treat the metal with a phosphoric acid etch or Naval Jelly; and follow up with a urethane sealing primer and top coat.

Treating rust with a phosphoric acid wash followed by a self-etching primer creates a solid foundation for top coats.

Pros: The process creates a surface that can be sanded and top coated easily, meaning it can be used on external surfaces like fenders and doors. Since the top coat is traditional automotive paint, the color choices are unlimited.
Cons: This process requires more than one step from start to finish. It also calls for professional equipment, including a mask or respirator for the priming and painting stages. Future rust prevention is dependent on the quality of the top coat and surface prep, as success hinges on completely removing all of the rust.

Ideal: Metal Replacement

If the sheet metal contains holes or is very thin, most experts say that the proper repair is to simply cut away the rusty sections and weld in fresh, new steel. Much like removing cancerous tissue in the human body, cutting out rusty metal will nip the problem in the bud and keep it from spreading.

The ideal way to stop rust is to remove it completely. Start by grinding away the rust with a wire brush and abrasive wheel. If there are holes, cut them back to good, solid metal and fill them with fresh steel sheet.

Pros: This is probably the best way to completely remove all traces of the problem. Instead of using chemistry to turn rust into an inert substance, the rust is simply replaced with new, solid metal.
Cons: Replacing steel can be expensive and time-consuming. Bare steel will still need etching and sealing primer as well as a top coat to prevent new rust formation.