Last wof I got a pass but a note on the sheet about rust on the boot floor.

Anyway, have sanded it and primed it with red oxide primer, will sand again once I see how rough it is over the worst part and then paint it with a hammered finish charcoal coloured paint as I have been advised that is sufficiant to get thru as it was just a warning and not a fail, the thing is the underside is looking a little bit like it needs some TLC under the spare wheel well. Whats the best to put on the underside of a car that will cover the surface rust and not just chip staight off?

I was thinking of covering it with some black plastidip spray can. I only have short axle stands so getting right under the car isnt really that easy, but I could get in and give it a light hand sand and spraycan without too much drama.