While I had the BMW wagon in the garage replacing the rear axles and sorting out the turbo control vacuum issues, I noticed the start of some corrosion in critical areas - on the steel body near the rear subframe and brace mounts in particular.
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Rust bubbling the paint near the spare tire well - frame stiffener mount
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I had some time so I decided to take action now to avoid larger problems further down the line. Two years ago, when I did
the turbo replacement project, I also dropped the differential to give it a thorough cleaning, replace all the seals, and paint the steel body. At the time, I used Tremclad spray paint. After 2 years, 2 winters on the road, the rust was coming back and I was almost back to where I started. From some research on the internet, I found the POR-15 system - a polyurethane based prep and paint system that seemed to be much more durable, so I decided to give that a try.
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Rust returning to the differential case - 2 years after Tremclad application
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The first component I cleaned up and painted was the right front bearing carrier. I had the drive axle out of the car
to repair the oil leak, and some of the suspension components disconnected, so this was a good time to access the part and get it done.
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Bearing carrier - loose rust removed, degreased, and treated with the rust conversion primer.
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The POR-15 system is comprised of a few basic steps:
- Remove loose rust
- Degrease using the POR-15 degreaser
- Prime using the rust conversion primer
- Paint - at least two coats - using the polyurethane paint.
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Bearing carrier painted with POR-15 Black Paint
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What's interesting with the system is that the degreaser and primer are both water based - and the paint required humidity to cure. This allows you to perform the steps fairly quickly one after the other. You don't need the part to be perfectly dry before painting (although you don't want any standing water on the part which may affect curing by causing bubbling under the paint).
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Surface rust on the rear bumper
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| More surface rust on the rear bumper |
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Rust on right side rear bumper mount
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I had pretty substantial rust on the rear bumper, trailer hitch and bumper mounts. Large areas of paint were flaking off the trailer hitch, and large areas of the rear bumper were rusting badly.
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Loose paint and rust removal with 3" carbon steel wire cup brush on grinder
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I pulled the rear bumper and mounts from the car, and the trailer hitch. I used a 3" cup brush on my Dewalt 20V grinder to remove loose paint and rust from the parts. It took about an hour to do all the parts - there was substantial rust and the cup brush worked fantastically. What a tool, highly recommended.
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| 3" Wire Cup Brush - great tool for cutting through paint and rust |
With the parts cleaned, I washed them with soap and water to remove all the loose scale and grinding dust, prior to degreasing.
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After loose paint and rust removal, rinsed and ready for degreasing.
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To degrease the parts, you use the POR-15 degreasing solution, diluted 4:1 with hot water. I use a plastic brush to scrub the parts and get into hard to reach areas. When complete, rinse the parts with hot water.
Immediately after rinsing, you can do the rust conversion primer since it is water based. Apply the rust conversion primer to all bare and exposed rust and metal. I use a 1" paint brush to scrub it in as good as possible. The instructions suggest you leave the surfaces wet for at least 20 minutes, I went back and reapplied the primer about 5 times over the course of an hour to ensure a good application.
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| POR-15 Metal Prep and Degreaser |
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| POR-15 Rust Conversion Primer applied - it's slightly foamy - keep it wet for the entire treatment time |
Once the rust conversion treatment is complete, rinse the parts off with hot water and allow to dry before the paint application.
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| Bumper surface following rust conversion primer treatment. |
The POR-15 coating comes in a limited number of colours for various applications. There is Gloss Black, Semi-Gloss Black, Silver, Gray and Clear. In addition, there is a high heat caliper paint available in Black, Blue, Red, Silver and Yellow. I have the Silver, Gloss Black and Silver caliper paints. For touchups on the underbody steel, I used my silver paint, just so that it wouldn't contrast too badly with the seafoam green factory color. For the suspension parts, rear bumper and hitch, gloss black obviously from the photos.
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| POR-15 Gloss Black applied to the trailer hitch |
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| POR-15 Gloss Black applied to the rear bumper and mounts, and trailer hitch |
The paint application is pretty straightforward - I use disposable brushes so I'm not bothered with cleaning them. You need a minimum of two thin applications, applied before the coating cures completely. You can recoat once the initial application is dry to the touch with a slight finger drag (stickiness). If you allow the first coat to cure completely, you need to sand before the second coat - it's important to time your work.
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POR-15 Silver applied to rust touchups on the underbody (battery box next to suspension compressor)
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I wasn't able to get up around the rear suspension mounts with a tool to remove loose paint, there is not a lot of access in those areas. I've ordered a dremel flex extension - will see if that allows me to get at the flaking paint in those areas and I'll post an update when I get that done. I really wanted to get all the corrosion protection done before this winter.
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Gloss Black POR-15 on the rear differential case.
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I painted the rear differential in the car. The Tremclad provided some protection, so I didn't have massive pitting and flakes of rust, so prep was pretty quick and easy. The differential is exposed to road salt and grit, so I'm looking forward to how well the POR-15 holds up over time.
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Reinstalling the bumper and hitch components - spacer washers for bumper held on by masking tape
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It was totally worth removing the hitch to treat the rust on the hitch - there is no way I could have done a proper job of it on the car. It's common to see rusty hitches - but I hate seeing the rust on my car. If I'm going to keep it, I want it to look good.
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| Ready to lift the trailer hitch back onto the car |
Reinstalling the hitch was fairly straightforward. I treated all the hardware by soaking it in the POR-15 primer overnight. Once everything was reassembled, I coated all the fasteners with the black POR-15 coating.
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I didn't completely disassemble the hitch, so I needed to pull the crossbar into position with a load strap
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| Rear bumper and hitch installed. Looks great in gloss black - will mostly be covered by the bumper cover. |
So - another job done. Looking forward to seeing how well the POR-15 holds up over time. The final step was to apply fluid film in critical areas - around fasteners, where there is metal to metal connections, and inside the rear bumper cavity that was inaccessible for painting. One other comment - removing the rear bumper cover on the E61 is really quick and straightforward, much simpler than removing the front bumper cover. It only takes about 10 minutes, and it's probably worthwhile removing it periodically just to clean out the road salt and grime that collects within the rear bumper that caused a lot of the corrosion in the first place.
Sources
Some POR-15 products are available at Canadian Tire at a reasonable price. For everything that Canadian Tire didn't carry - I simply ordered from the POR-15 online store - quick delivery in a couple of days. Real simple.
Let me know if you have any comments or questions below.
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