I wash my van weekly with wfp, not the same brush as i use for windows, plus don't use it on the wheels, it comes up a treat every time when rinsed off. Saves drying with a chamois leather
As for washing in direct sun light, a friend comes and valets my wifes car every now and again, IN DIRECT SUN LIGHT SOMETIMES, never had a problem but he doesn't use fairy
Darren