I work on the basis that a layer of dirt keeps the bird poop from damaging the paintwork

I previously worked on that theory. The paint lasted about 5 years before the clear started to peel. Dirt, dust and especially bird poo and egg white eat into the paint work. ______________________________
Vogue tdv8 08MY poverty pack - wow

Mine gets laned sometimes so i tend to leave it cake layered and because its Bonnatti its not that bad really the rain washes it off mostly at the minute  FFRR MY06 facelift With TDV8 Alloys Zeros/ATR's
Mantec Sump Guard, Rigid Load liner, MY10 BT upgrade.

Mine are usually rain washed as well. And once, maybe twice a year they are treated to a car wash. But that mostly for the hot wax spray (that I used to disdain) because it works like rain-x on the windshield. Life got in the way and I no longer clean my cars regularly...  
RRC 2Dr, RRC 4Dr,
P38, and 2 L322s
(wife thinks I'm nuts - prob right, too)