Hey josho, what do you mean with "discolour the painted surface around it". Are you saying that 1. the base paint became covered in wash colour too much, or 2. do you mean that the base colour is "gone completely" ?
Im no expert myself having built only 2 kits so far, but for the first case, it could be becasue your wash is too "thick". You want to have a tinted thinner, not a thinned paint.
For the second case, umm....if you use enamel for the base, you might try using arcylic for wash, or vice de versa.
Personally, i use emanel wash over enamel base. But i usually wait until about 3-4 days after base colour is applied. Then i use natural turp (or I also tried mineral spirit, and it worked as nice) instead of enamel thinner. It should not "attack" the enamel base colour if you put it right. It is better to have several light washes, than to have one big wash.
Hope that helps.