The underside is very clean (have been told this by a number of people) and want to keep it that way so fully undersealed the whole of the underside and all the arches with...

2x : Hammerite Waxoyl Rustproofing Aerosol Black
1x : 2.5L Hammerite Underbody Seal Black with Waxoyl

This is mucky stuff and solvent based, I used the aerosol to reach the akward places.
Overall took around 3 hours, was cautious to not get any on the exhaust \ exhaust heat shield \ gearbox \ running gear \ coilovers etc