How can you thin Revell Enamel paint?

I want to be able to thin Revell Enamels to an acrylic like viscosity, as even if I wash my brush after using the paint, it sticks to the brush and is only removable with white spirit. Yes, I could keep using the white spirit, but it's quite annoying and unecessary. Alternatively, can anyone suggest a good (cheap!) acrylic paint range for model painting? Thankyou for any feedback.