Prob just as easy to get some chassis black/industrial black in an can and spray as needed.
If you really want to get in there and clean everything out I'd suggest getting some under body shultz 'which will require a special type spray gun and a compressor', and reprime it 'this will also rust proof it as it is a thick liquid'.
You can paint this stuff on as well, just depends on how even you want the finish 'it wont be disastrously uneven just the typical hand paint v spray finish 'which you will be able to even out as it dries'.
Depending on the thickness of the schultz chosen you may have to thin it out a little 'with thinners/petrol'.
Note: Using schultz will definitely be an on the grass job as its a nightmare to clean off your hands let alone brickwork/concrete etc
