I think Green Dream is talking about the underside.....
Plenty of elbow grease, lots of baths and showers to get it all out of your hair and eye's etc.....
Sersiously though, brush off what you can with a course brush, then degrease with something like white spirit on an old rag.
For the arches I then re-undersealed using Hammerite Underseal. This tetroseal one is too thick and sticky.
Then I coat the underseal with Silver Smoothrite. Although I don't think you will get smoothrite in 'Mica Red' so you may have to just use black.
Where possible remove and repaint (Plasti-Kote is quite good) any fittings/brackets or even get items powder coated.
Refit with new fixings if possible, stainless nuts and bolts are quite cheap on ebay.
When removing sticky/rusty items don't do what I've just done and snap an M8 bolt that holds the rear ARB mounts to the chassis rails
. Need to now drill it out an re-tap it (hoping it goes to plan).