I used a rag on the end of a telescopic pole with Xylene soaked in it. I went from both the engine bay and from the vents. Worked fine. You will want to remove your battery and mask off any paint you don't want to mark.
The solvent will depend on the paint that was on there. Xylene is good on most stuff (branded as Xylol and in a lot of brush cleaners) as is acetone and mineral spirits. Just pick something as aggressive as you want to be. If you just want to slightly etch it, hit it with mineral spirits and then a grey/red Scotch pad / wire wool. Depending on the paint you use, primer will cover up to 120 grit scuffs and basecoat around 300-600 grit. Then again it is in an area no-one looks at, so you could wing it.
Thanks for the advise, have you ever used ferro betol ? I used some on my tinware and wiped down with thinners after it dried. Then painted a direct adhesive one coat industrial 2 pack 70% gloss on them. Just seen a few bits after months of storage and there has been a slight reaction :x Some of the paint has bubbled a bit, well piss ed off. I should have stuck to cellulose and enamel paints.
The guy who abandoned the Resto Project we bought cut them out so they could be painted on the inside. Have to say there was a significant amount of rust on them down deep where they are difficult to access. Been cleaned up, treated, etch primed and primed. Thinking of coating with smooth Stone Chip before final painting.
Thanks for those pictures, I will think about that. Mines got the extra insulated skin inside so it would cover up easy if I had a removable panel underneath to get inside vents. What would be the best way of cutting it out without removing too much metal so I use it to cover the hole again.