Little update, as mentioned the inner rear wheel arches were a bit scabby. So I cleaned them up with the wirebrush on the angle grinder. Then gave them a quick coat of primer.
Looks like I’ve caught it just in time, much more and I would of been cutting holes in the arches and welding plates there too. Later on once I have the fuel tank out I’ll go over the whole underside and paint any patches that are starting to corrode. Then I’ll underseal and waxoyl certain sections