Project E36 M3- Battery Compartment Rust Repair April 29 2021

Our E36 M3 had a leaking battery, and this led to some rust in our battery box. This was despite the acid-absorbing and neutralizing mat that we had placed under our battery. We set out to assess the extent of the damage and then fix it. Fortunately, the rust wasn't too deep and was able to be addressed with some rust treatment paint. POR-15 is our go-to rust treatment paint, and we've used it several times before with great results. It is incredibly tough and bonds tenaciously to the substrate.

BMW E36 M3 battery compartment with rust.

This is what our battery box looked like before we did anything. You can see the obvious rust over a significant area.

BMW E36 battery compartment after scrubbing rust with a stainless steel brush about the size of a toothbrush. Rusty areas are now dull metal.

We first needed to get rid of the rust. We started with a small stainless steel brush with thin bristles, and it was able to get into the crevices. If you have more extensive rust, a larger brush or even a wire wheel on a drill might be a better tool. We ran a vacuum while brushing to collect the rust dust.

With the rust gone and the bare metal exposed, it was time to commence with our POR-15 treatment kit. Here is a link to a kit that has the degreaser, etcher and paint in small amounts: POR-15 kit*. It contains more than enough product to cover an entire battery box, and has all the brushes and stir sticks that you will need for one job. 

The first step is to degrease the area. The Marine Clean product calls for rinsing the area with water after applying. Since it is difficult to extract water from the battery compartment, we opted for Simple Green. We wiped it up afterwards with a wet towel and then a dry towel.

Bottle of POR-15 Metal Prep.

The next step was to etch the bare metal so that the POR-15 could have good adhesion to the substrate. Metal Prep was brushed over the bare metal areas. The metal should be wet with metal prep for at least 30 minutes. We brushed more on every 10 minutes. Afterwards, we need to rinse with water, but again, this area is not conducive to rinsing, so we alternated with 3 rounds of wet and then dry towels.

Battery compartment with bare metal etched into a dull finish.

When etched, the metal goes from shiny to dull. Before applying POR-15, the area needs to be completely dry. We used a heat gun to speed up the drying and then let any remaining water dry up overnight.

Can of POR-15 Rust Protective Treatment.

Finally, we're ready to apply POR-15. We poured a small amount into a separate container, because double dipping a brush into the container contaminates the product, according to the manufacturer.

Battery tray with 2 coats of black POR-15 Rust Protection Treatment applied.

We brushed on two coats, 5 hours apart. Thin coats are the way to go.

Can of touch up paint from TouchUp Direct.

Before the paint fully cures is when to hit the POR-15 with a top coat if you want to top coat it. We masked off the surrounding area and put plastic bags over the battery cables. We got a can of color-matched spray paint from TouchUpDirect. Because this is the inside of the car, there is no clear coat on the factory finish, so we omitted the clear coat for this job.

Battery compartment after spraying touchup paint.

We were impressed with the nice fan spray pattern of the can and the nice, light volume of paint it put out. Especially for a tight area where it might be hard to keep the can moving quickly, the can put out nice, light coats, which prevents runs. 3 coats of paints was enough to cover up the black POR-15. We are not sponsored by TouchUpDirect in any way.

Note that the brushed-on POR-15 doesn't have the smoothest surface finish. For a more cosmetically sensitive area, you might consider spraying on POR-15 with a HVLP gun, or spraying whatever primer you can find in a rattle can. However, POR-15 is one of the toughest coatings that there is, and one of the best coatings in our research and experience for stopping and preventing rust.

Battery compartment with Optima Yellow Top installed.

Lastly, we switched to an Optima YellowTop battery. This is the exact model we used, and this is the equivalent RedTop version without the deep cycling durability of the YellowTop. These are absorbed glass mat (AGM) batteries that don't have acid sloshing around inside. They can't spill and wreck havoc in your battery box. Lastly, the YellowTops can be deep cycled more times with minimal performance degradation. This means that any time your battery dies, you will likely be able to recharge it and put it back into service. It's a more robust battery with quite a few benefits over stock. Since the Optima battery doesn't fit in the E36 without modification, we developed a lightweight mounting adapter that you can find in our store here.

Our battery compartment looks close to new again, and the rust shouldn't be back any time soon. Thankfully we were able to catch the damage in time before it got too bad. The job wasn't too difficult, and we were pleased with the end result.

Related Articles:

E30 battery compartment with drill and wire wheel attachment.

BMW E30 battery compartment rust repair

Optima Yellow Top battery installed in an E36 M3.

BMW E36 Optima battery mount adapter

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