"In this Special Series, I document my complete paint & body restoration along with the exterior upgrades to CSL specifications. 

In this entry, I go down the rabbit hole. The underbody gets dismounted and we get disappointed by BMW once more. 

We revise our plans, strip the underbody, put money where it matters and discuss durable coatings. Lastly, in an inspired decision, I wink at the E46 M3’s past: I paid hommage."
-Matt

Down the rabbit hole

In one of the first entries, I answered the infamous painter's question. I describe in depth what OEM paint mean, and how BMW had been cutting corners on paint forever. 

My goal was to do OEM+: a proper engine bay and underbody color matched and clear coated.

As we wrapped up painting the shell, we got the car on the lift to plan the underbody. 

We started stripping the underbody of its front and rear subframes, suspension components, diff, fuel tanks and bracings.  

I had a new diff cover fitted this summer, but the casing itself was the usual rusty BMW diff. 

New RTAB monoballs, but the arms themselves were also rusting away, shedding its black coating. 

When doing the solid subframe bushings, I bought a used subframe to pre-press the bushings and discard the one on the car previously. 

This new subframe was in good condition - appearing to have been made in 2010. 

The front subframe came out as well. Here is it mounted on the car. It's pretty much a bar and that's that - you start to understand why these cars were light, and raw. Chassis sophistication wasn't all that. 

[...] still, BMW further disappointed on the underbody. 

The entire underbody is on a grey primer looking coating. It is not body color.

 

A quick inspection of the subframe*

You can't chat underbody without chatting subframe - or rather than rear axle carrier panel where the subframe actually mounts. All 4 mounting positions had been previously reinforced.

Whoever did the plates welding didn't apply any coating afterwards. The bare metal was left to rust over the years. We stripped all 4 down fully to inspect, it was structurally sound. 

Here is one of the front mounts after being lightly sanded down, getting ready for its new coating. 

I toned down my plans

Seeing BMW's original underbody, I immediately felt a body colored liner would be trending too much towards a show car aesthetic. 

I already had a sense of how it would have looked, having already completed the front wheel arches in a body colored liner during the engine bay spray session. 

[...] and put money on durability first.

We decided to go for a more subtle yet contrasting black liner for the underbody. My goal was to set a dark canvas for other components to be highly visible. 

The freshly painted shell got masked and ready for coating. 

The coating was evidently done out of the booth - a more common approach to rough coatings. They don't require a sealed environment. 

The rear subframe mounting points that had previously been reinforced were fully coated to seal in the welds and raw metal. 

You can see where the separation between the body colored engine bay liner and the underbody.

This is all getting covered up with mats and shielding, along with the transmission - still, it feels good knowing it's done right.

Durable coatings can look good though. 

Zinc Coating?

As the guys tore into the underbody, we chatted about zinc coating the bolts to get the best looking underbody there was. Zinc coated bolts are my go-tos when replacing underbody bolts.

They last forever and look great - but here we wanted the focus elsewhere.

In the context of making a drivers' car, I did care about durability and overall aesthetics of suspension and subframe components. 

The aluminum suspension arms were sandblasted before being coated with something other than paint. 

The diff also was sand blasted in-house. This will require a few diff fluid flushes probably to ensure no sand is stuck in the diff. It's getting rebuilt later on anyways!

All suspension components got sand blasted or sanded manually, including the spindles, wheel hubs and brake shields.

Exaggerations? You get to judge. 

Marine Grade Sprayable Zinc!

We chatted with his BASF rep a week earlier and he suggested one of these marine zinc sprayable coating. It would improve durability massively while retaining a more paint-like appearance as the sprayable zinc is smoother than liner, and has a silver color by default. 

It retains the aluminum appearance of suspension components that already were aluminum. 

It gives an aluminum appearance to those that weren't. 

Gone is the flaking black coating! The diff casing now matches its cover.

The rear axles also got the treatment but for a reason a few of you already know. They are a total PITA to remove from the rear spindle for most owners living in rust prone environments. We faced those same challenges on the M3 V8. We preferred not to risk it. 

The rear trailing arms's coating was flaking due to surface rust starting to eat into the original coating. It hid away this little detail from BMW M. 

The subframes staged the scene.

The black underbody and silver suspension components set up an underworld stage. I wanted to do something special here. 

The front & rear subframe got sand blasted and ready for paint.

I paid hommage, I painted the subframes in Phoenix Yellow.

I felt motivated by personal events and inspired by a famous quote from the great writer Oscar Wilde. 

"- What is done is done. What is past is past.

- You call yesterday the past?"

- What has the actual lapse of time got to do with it? It is only shallow people who require years to get rid of an emotion.

A man who is master of himself can end a sorrow as easily as he can invent a pleasure.

I don't want to be at the mercy of my emotions. I want to use them, to enjoy them, and to dominate them."
- The picture of Dorian Grey

The rear subframe bracing got coated in Zinc instead. 

Up next, time to finish the paint process on all remaining parts, and take a detour on engine work.

We bulletproof the S54 and we upgrade the cams.

We'll be back to chat on permanently bulletproofing the rear chassis of the E46 afterwards as we start seeing the home stretch

[...] or will we ever?