Treatments for Rusted Metal: Rust Bullet, Cosmoline RP-342, and Others

This past month I had to deal with a hole rusted through the body of my old but serviceable car. A local body shop told me it might cost more than the car is worth to replace the rusted parts. The rust does not look like it has compromised the vehicle’s mechanical integrity (a real concern, since today’s cars rely on the body sheet metal for strength), but I don’t want it to go further.

Being a (retired) chemical engineer, I read up on ways to deal with rusting metal – especially how to arrest the progress of rust. Here I will share what I have learned about several types of coatings. I hope this information is of use to some readers.

Rust Converters or Reformers

There are several classes of coatings which chemically react with the rust on the surface to stabilize it. In all cases, it is assumed that the user removes all loose rust, but that some rust remains on the surface to be treated. This YouTube video has a detailed comparison among several leading brands of rust converters.

Most products that have “Rust Converter” or “Rust Reformer” in their names contain tannic acid. This reacts with the soft red rust (iron oxide) to convert it to a more stable black iron tannate. This is fairly hard and smooth. You can then paint over the converted surface. Rust converters can handle heavy rust, but often they will do very poorly if applied over non-rusty surfaces. In some cases, you can apply them to mixed rusted/bare metal/painted surfaces, and then you must clean the unreacted converter off the non-rusted surfaces. Each product is different, so you have to dig into the directions for each specific product.

Another type of rust converter is phosphoric acid. This reacts the rust to a stabler iron phosphate. You need to promptly apply a top coat of paint to then seal the surface.  A popular phosphoric acid converter is Ospho. This seems easy and inexpensive to spray or paint onto large outdoor objects like iron fences or cars. Again, you need to read directions for your particular product, to see if it is compatible with non-rusted surfaces, and whether you need to clean off unreacted product after treatment.

Another type of rust conversion product contains diisocyanates, which dehydrate the surface rust, and ultimately form polyurethanes.   POR-15 and Rust Bullet coatings both contain diisocyanates. These products are painted on, and form tough coatings. One coat of POR-15 over rusted metal gives a very tough coating. Some disadvantages of POR-15 are that it will not stick well to non-rusty metal (unless the metal is etched), it is not UV-resistant, and it is tricky (though possible) to top-coat it with other paints. Rust Bullet will stick to nearly anything, but it takes a minimum of two coats to seal a surface. It is UV-resistant, so it can be a top coat. It can easily be top-coated if this is done on freshly-cured Rust Bullet, else you have to sand the Rust Bullet to get the next coat to stick. (Make sure to wear rubber gloves, because any Rust Bullet or POR-15 that gets on your skin is hard to get off).

Rust Encapsulators or Sealers; Rusty Metal Primers

There are there are paint-like products that claim to do a good job sticking to the rusty surface and penetrating through the rust, down the metal surface to seal out further water. Eastwood Rust Encapsulator is a well-known brand. I would include the many “rusty metal primer” spray and brush-on paints (e.g. from Rustoleum) in this category. Some of the primer paints include zinc to presumably give something of a galvanizing protective effect. Typically these products can be applied to both rusty and non-rusty surfaces, and can be readily top-coated.

Rust Preventative Coatings

There are many types of paints and other coatings that may be applied to non-rusted surfaces to help prevent rust from starting. These are popular to apply underneath a car or truck. There is a good, wide-ranging discussion of these various coating at Entry #14 ( by “pro10is “ ) at this Ascent Forums link. A product that caught my eye there is Cosmoline RP-342. This sprays on like a medium-thick oil, then hardens into a wax-like coating that should do a good job excluding water from the metal surface. On my car, I ended up using Rust Bullet on the rusted exterior surfaces, and spraying Cosmoline into interior sheet metal chambers to retard further rusting. (If I had known about it in time, I might have chosen Eastwood Internal Frame Coating instead of Cosmoline for spraying the interior spaces.)