Battery compartment surface rust; how would you treat this area?

[WoGGo]

Some time ago water was getting into this area via the perished tail light seals.

I need to change my battery so I'm looking to address this issue whilst the battery is out.

I'm not looking for perfection, but given I have no experience in this area, what would be easiest/simplest way to fix this and prevent further rust from spreading?

I have a brand new battery tray and nuts if that is relevant.

[Rokoz]

Wire wheel/grinder to remove all of the rust and paint. After it’s completely clean use epoxy primer and paint of your choice SEM 62233 is a pretty good match for the green.

You will want to remove those metal plugs and get some plastic ones

[Big Nodge]

I’m dealing with this on my e36 and am planning to use POR-15, it’s a little more forgiving on the rust removal aspect.

[Cubieman]

I ground it down really well and used POR-15, did this on the bracket as well.

2K epoxy aerosol primer if you want a quick and durable primer coat before the paint.