[Raurik]

Did you water down the wash a little more? Adding a little water to it may get the streaks out better, also make sure the black paint is completely dry, because putting wash on it can make the paint wet and that may also contribute to the streaks. When you wash you wanna use a relatively soft brush to limit the streaks as well. (posting pics always helps giving advice, but your description was fine if its what I'm thinking it is heh)

Good Luck, hope this helps!

 

[Raurik]

Did you just basecoat/prime it gray or did you just put the wash on the plastic? thats probably the #1 reason it came out streaky, The plastic is so sleek there's nothing for the wash to stick on. So a prime and basecoat on the model definitely helps.