BMW E30 Battery Box Rust Repair Guide | How To Fix Rear Battery Box Rot With Welding | 016

BMW E30 Battery Box Rust Repair Guide | How To Fix Rear Battery Box Rot With Welding | 016

In this BMW E30 restoration episode, I tackle the typical BMW E30 Rear Battery Box Rust.

This is an absolutely classic place for these old 80s 90s 3 series cars to rust through. Due to the design with the sunroof vent, its inevitable that water ends up getting in the boot, in that box, sitting there for years and eventually rotting through that thin skin of metal.

Here's what I use in this video:
Clarke Weld MIG 150 TE Turbo: https://ebay.us/jbdfE8 (this is the newer version)
0.8 Welding Wire: https://ebay.us/eQWqWx
Argon/CO2 Welding Gas & Regulator: https://ebay.us/7BOngD
Angle Grinder Wire Brushes: https://ebay.us/sSJjtM
Dewalt Angle Grinder: https://ebay.us/CmsNuW
Safety Goggles: https://ebay.us/2Dvt5C

My E30 project is actually a 316i so the battery is situated at the front under the bonnet. The front battery box is also a typical E30 rust spot, but I'll get to that at a later date..

Batteries were situated in the boot for the 6 cylinder cars which is why its referred to as the rear battery box.
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To tackle this repair I used my trust Clarke welder, some argon/co2 mix welding gas, some 0.8mm mild steel welding wire, and of course some new sheet metal, in 1.2mm to be exact.

I started off by using a wire brush on an angle grinder to clear away the rust and see exactly what needed cutting out. Once the rust was cleared I found an additional small rust hole which I decided to repair while I was there, adding a notch to the top of my cut-out.

I cut the rusty metal out using a cutting disc on an angle grinder, and had to cut the metal in two pieces rather than one neat one. With that removed, I used the old masking tape trick to get a better template of the patch I needed to cut.

I then set about cutting my patch to weld in from the sheet metal. This turns out to be one of the hardest parts of any rust repair, especially when its a more complex shape with multiple bends in it too.

After a lot of back and forth to trim and refine my patch, I did the main bend in the metal using my vice and a hammer to give me a relatively clean and straight bend. I was quite happy with it after all.

As the inner part of the battery box (facing the spare wheel well) was also rusty, I'd cut that out too and opted to weld a new piece on there first, the idea behind that was that I could butt my main patch up to it to reseal the battery box off and prevent future rust.

Once that was welded in and cleaned up with the angle grinder, I moved onto welding in the main part of the repair. Using a couple of handy welding magnets to hold the repair piece in place I set about getting a few tack welds on which was successful.

The method for welding thin sheet metal body work on a car like this is to work around tack welding everything rather than trying to lay down beads. I'm far from a welding expert, but with my Clarke MIG welder set on its second to lowest setting, I was getting good penetration and the tack welds were laying down very nicely.

A pro welder would be able to do this without building up so much weld, but knowing I could come back and clean it all up with the angle grinder later, I set about making sure there were no gaps in my weld.

To top it off, I also welded from the inside to make sure everything was tied in solid from both sides.

Once done, I allowed my welded bodywork to cool down before using a sanding flap disc on my angle grinder to knock all the welds down smooth. I then panel wiped and threw on a coat of primer on to prevent flash rusting in the meantime while I do more work on the E30.

As you can tell from the footage, I'm very pleased with the end result, if I'd paid a bodyshop to do this professionally, and they came back with the same result, I'd be very happy with the job, which is a very good sign.

This rust hole repair success gives me confidence to move forward with the project and take on the next big bodywork issue - the wheel arch.

For more helpful how-to guides and restoration project logs, visit our blog: https://www.spannerrash.com/

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