On a side note, is it just me or does the shape of the tailgate mean it picks up more dirt than normal. I swear I'm having to wash my Mk4 way more often than my Mk3.
Just noticed yours was dirty in the same places as mine.
A dirty rear end is a consequence of aerodynamics and airflow over ‘two box‘ shaped hatchback and estate cars. My understanding is that as air moves over the surface of the car when it’s being driven forward, an area of low pressure is created immediately behind the car, and air containing dirt and dust particles in the atmosphere are pulled in to fill that area of low pressure. That dirt and dust swirls around and gets sucked onto the rear facing panels (bumper, tailgate, rear screen).
As for your mk4 getting dirtier than your mk3 - it could be that the mk4’s body shape creates a slightly different airflow pattern over the car compared to your mk3. Also, some paint colours will show dirt more readily than others, so if the colour of your mk4 is one that shows dirt more readily than the colour of your mk3 then that might also
help explain why your mk4 gets dirtier and needs washing more often than your mk3 did.
I’ve read on various forums of people fitting rear mud flaps in the expectation they’ll keep the rear of their car cleaner. Due to the way dirt gets attracted to the rear of the car, mud flaps will make pretty much no difference - they may
help reduce stone chip damage to the underside of the rear bumper though that some mk4 Leon and
Cupra Leon owners have experienced.