Your Miata Could Be Rusting

By Harry Yates

Your Miata is made fairly well, encompassing the latest in materials, and design techniques to produce a strong unibody convertible. Mazda kept the weight down for optimum handling, and performance, but this requires a little extra care on behalf of the owner to keep the car looking good for many years to come. With very thin paint, and sheet metal, the body of the Miata is susceptible to damage.

Millie is a 1991 model that currently has 126,700 km on it, and is in exceptional condition. I have gone over this car's undercarriage very carefully, and it is free of corrosion even though it was never undercoated or rustproofed. This is testimony to the fact that Mazda used galvanized steel. However, there is a major design flaw that could be hiding rust on your beloved Miata. Fortunately, this is easy to check, and to repair if caught in the early stages.

The area in question is the lower rear part of the front fenders. I read about this on the Internet (I belong to the Miata Listserv), and decided to check out our car. At the bottom of each fender, there are two 10mm bolts that hold the fender to the rocker panel. It's a tight fit to get the socket over the bolt heads due to the factory gravel guard, but just push it on, and remove these two bolts. You will also want to remove your front mudflaps if you have them by using an 8mm socket or wrench. Also remove enough of the plastic fender liner so that you can get your hand in between the fender and body.

With the mudflaps removed, and the fender liner moved out of the way, you can gently pull the fender away from the body of the car. You will be amazed as to the collection of dirt, mud, and gravel that has collected at the very bottom of the fender. I used an old toothbrush to scrub the dirt away from the bottom of the fender, and the rocker panel. I then washed the lower part of the fender from the body crease where the gravel guard starts to the very bottom edge with an old wet chamois. I had to do this about three times to get the area clean. I then thoroughly dried the area. With all the dirt accumulating in this area, corrosion can easily start as the dirt will retain moisture. I highly recommend that this area be cleaned and inspected on an annual basis for corrosion. Now, if you car looks good after cleaning, you can spray in some rustproofing material, put on some anti-seize compound on the bolts that hold the fenders on, and reassemble everything. In our case, we needed to do some rust repair.

With the fender dry, I could see all these little bumps on the inside of the fender. Upon closer inspection, I noticed that the bumps were rust blisters. The paint had not yet broken away from the rust. Oh, oh! Out comes my emory cloth. These blisters were distributed throughout the horizontal part of the fender where the bolts attach to the rocker panel, and a bit on the curved part of the panel. Basically, the area of sheet metal that was infected was below the black paint on the fender. I sanded this entire area down to bare steel, but there were many dark spots present. This is from rust that has taken a deeper hold on the steel. This was bad news as these dark spots will cause the rust to come through the steel in time.

There are a few products available that will penetrate the rust, and arrest it. I used a product called Rust-Mort. These products are available at automobile paint suppliers as they are very specialized products. I bought a litre jug of this stuff, but try and get a smaller jug if you can. You don't use much. Rust-Mort is made by SEM. It converts rust to a black or grayish, hard, insoluble coating. This is powerful stuff. It contains 75% phosphoric acid, chromic acetate, isopropanol, and water. You MUST wear gloves with this stuff, and use in a well ventilated area. This product is very easy to use. I poured about an ounce of this stuff into a glass jar, and used a one inch paintbrush to apply it. You give the effected area four thin coats of this stuff, waiting a few minutes for it to dry before reapplying. After the fourth coat, you leave it on for 24 hours. After 24 hours, you just rinse the treated area off with water, and wipe it with a damp chamois to properly clean. The metal that has no rust will be shiny and clean. The areas that Rust-Mort converted will have black spots. After rinsing, and cleaning, dry well. This is imperative. I let the car stand overnight before I did more work.

The next day I used a self etching primer to prime all the bare steel. I tried to spray it into a cup and use a small paintbrush, but I wasn’t getting an even coat of primer this way. I ended up covering all surrounding areas with newspaper, and masking tape, and spraying the insides of the fender. I sprayed in three coats of primer, and three coats of black acrylic enamel, then two coats of rubberized undercoating as a rustproofing layer. I used Mar-Hyde primer and undercoating. The four bolts that hold on the fenders where hiding quite a bit of rust under the gravel guard layer. After removing all the paint and gravel guard, I treated these bolts with Rust-Mort as well, and then after bolting the fender on, I etch primed, painted, gravel guarded, and painted over the bolts.

Due to the fact that corrosion was present on our car, this added many more hours of work. Our car was out of service for an entire weekend. For regular cleaning, this would be a job that would take about two hours or so. I would certainly recommend spraying in some rustproofing while the fender is unbolted, and use an anti-seize compound on the bolts before reusing them. Now, I must say that I am not a specialist in autobody repair. I've done this repair method on other vehicles that I have owned in the past, and have had success. Read all manufacturer instructions, and take the necessary precautions as outlined by them. Having said that, I hope that this procedure can help you and your Miata endure many years of happy ownership.

Last Updated: November 02, 1997