After spending a few more hours with the wire brush on the drill, I began to realise that it wasn’t working that well.

After a bit of net searching, I found a wire brush attachment for a grinder. A trip to the DIY this morning and I bought one along with a different one for the drill. It’s an evil looking thing.
I was worried how much damage it would do, so I had a go of it on the frame. Sparks, deafening noise and… Bare metal. Not a mark. No underseal, no rust. Amazing. Within a few minutes I’d cleaned an area that would take about an hour with the drill.

So onto the firewall. Another hour, and I’ve got shiny metal. compare it to what it looked like a week ago. It’s too cold to do anymore, and Im covered in cuts from flying underseal shrapnel. I’ll do some more next week when I get some spring compressors.