As a Mk1 owner I am surprised at how the back end stays fairly clean. This is a photograph I took today, after an 80 mile trip of mainly motorway/dual carriageway driving, with a few miles of country roads to make up the balance. It was a miserable day with filthy dirty roads, and tons of HGVs.
The car is dirty, embarrassingly so, but it has covered almost 3,500 of again, mainly motorway/dual carriageway driving since the last time it was washed. Compared to other vehicles I see on the road, the back end of the Mk1 stays remarkably clean.