Fuel tank pressure

Can someone explain how this whole EVAP/purge system works - my car (4.6 96my) does seem to have excessive pressure in the fuel tank - every fill up means I get a blow-back over clothing. I can't keep letting my wife fill up the car and get soaked with fuel (the price is ridiculous after all) so I was wondering what the problem might be.

Might this be connected to my car overfuelling and running rich?

Cheers

Look at Rave for the system fitted to your car, the control valve maybe duff not releasing excess vapour pressure into the engine when it should.