As
Neil S Has said i had to do mine Luckly for me i was @ a m8s workshop using his 2 poster removing the rear beam and noticed the rusted so cleaned it up and welded it whiles the beam was off
Does'nt look to bad tho sounded worse when you said it had Failed Badly on your other post
Fingers crossed Ps you see whot i mine by the wheel lines corsing more rusted got them out just in time
But buy a cuple of cans of Under Seal £2/3 an can will be better then just Paint IMO