Tech How-To: A Remedy for Rust
 Neutralizing and repairing non-structural tin worm on an older Mercedes-Benz

Article & Images Jay Hill

My hunt for a nice, rust-free W115 230 lasted for more than 15 years. It wasn’t an intensive search, but I did occasionally check the internet sites where my model of choice might be found. These cars are relatively rare: Only built from 1974 to 1976, the W115 230s are prone to rust in specific body areas because rear seals fail.

This past spring, I found a likely candidate: A W115 listed for sale in Bloomington, Indiana. The owner is not a mechanic but represented the car fairly. The back-rear window seal, trunk seal and taillight seals had deteriorated, allowing water to get into the trunk and under the rear seat. He took pictures at my request; it appeared most of the rust had not eaten all the way through the metal. I decided to take a chance that metal cancer hadn’t invaded the car’s basic structure.

Paying a professional to cut out and replace the areas of diseased metal would be too expensive to justify on a low-value car such as the 230. As many DIY enthusiasts do not have the experience to remove, replace and weld into place large metal panels, an option is to repair the non-structural affected areas with readily available compounds, especially if the car isn’t a concours candidate.

When treating rust, it’s vital to first identify and repair the cause and avoid further incursion. As a first step, I replaced two failed seals around the trunk lid, and then water-tested the new seals to confirm that no moisture was seeping into the damaged areas. I then went to work on the corrosion.

Getting down to the actual repair, I used a scraper to knock off all loose metal, paint and rust. I then mounted a wire brush in a hand-drill chuck and attacked the enemy head on, brushing until shining metal appeared. Next I used smaller wire brushes to remove – as much as I could – the rust in smaller crevices. I wore a good painter’s mask with a filter, goggles and heavy work gloves to avoid inhaling dust, getting dust in my eyes or cutting my hands on sharp metal. Then I swept up and vacuumed the rust particles from the areas I’d cleaned.

The next step was to remove all dirt, grease and grime from the cleaned areas. There are several products specifically designed to degrease and prepare metal surfaces for painting that are available in the marketplace – they all work well. Whatever product you chose, make sure to have proper eye, nose and hand protection; many of these preparations contain strong chemicals. Don’t forget ventilation: I used a huge fan pointed right at the area I was working on.

Having removed all rust that could be attacked manually, and thoroughly cleaned and degreased the area, I next used several applications of Rust Converter Ultra on the affected areas to chemically neutralize any residual rust and treat the bare metal.

To fill pinholes in the metal, I used POR-15 Patch Filler and Seam Sealer, applying it over the pinholes with a disposable paint scraper. Larger holes can be repaired by using POR-supplied fiberglass sheets, then spreading the compound over the fiberglass sheets; when dry, the POR compound is nearly as strong as metal. Be sure to wear disposable painter’s gloves – the patch compound is very difficult to remove from the skin.

After the filler and sealer had dried completely, the next step was to encapsulate the rusted areas to permanently keep out water and air: POR-15 is specially made for this purpose; other available compounds do the same thing. Once applied, it forms a water and airtight seal over the bare metal.

Warning: POR-15 must be handled with great care. Using the correct fume-rated chemical facemask, goggles, rubber gloves, long-sleeve clothing and ensuring proper ventilation is essential. If this compound gets on the skin, it cannot be removed; it must be left to wear off with time. I applied the mixture directly over the rusted areas with a cheap paintbrush. After use, the brush must be thrown away. I applied a second coat after the first coat was completely dry.

Be aware this product degrades in sunlight. After the second coat was dry, I applied two coats of Rust-Oleum Professional High-Performance Protective Enamel paint on top of the treated areas; I chose a color as close as possible to the factory original. When the paint dries, the rust repair is complete.

I then drained and removed the gas tank, cleaned the under body beneath the trunk and sprayed two coats of 3M Professional Grade Rubberized Undercoating under the entire area to discourage rust. I completed the entire repair by reinstalling the gas tank.

So, for the cost of a few hundred dollars in replacement seals, compounds and materials and a couple days of manual labor – while being cautious about ventilation and skin protection – I was able to repair nonstructural rust on my vintage Mercedes-Benz. I can now look forward with confidence to enjoying my W115 230 for many years to come.

Rear window, trunk and tail light seals can become dry and cracked, admitting water, leading to rust.

1. Detail of badly rusted area under rear seat before treatment.

2. The area after treatment. STEP-BY-STEP:

3. Badly rusted trunk.

4. After scrapping and wire-brushing, area was treated with Rust Converter Ultra to chemically neutralize residual rust, followed by POR-15 Patch Filler and Seam Sealer to encapsulate rusted areas.

5. To protect POR-15 rust inhibitor from sunlight, I painted the trunk with two coats of Rust-Oleum Professional High-Performance Protective Enamel.

6. The finished trunk.