Help, please: Failed washing a tank.

Hi all. On the weekend I got myself my first ever tanks. I've painted hundreds of other models (Orks, Marines and sisters), but never any vehicles (My technique worked fine on my dread, and I never think of that when I say vehicles).

The tank is a Black Templars Predator Annihilator. I took pretty much the same approach that I take with my actual marines; Black paint them, grey drybrush, black wash. It gets the highlights out, but leaves the rest of the model very black. I love how this looks on the marines.

Sadly, the wash on the tank hasn't worked out as well. Its covers up the drybrush perfectly, but has black streaks on the black. I was just using a small brush thinking that it wouldn't matter since the wash is very liquid, but its streaked where the brush touched the model.

I can try to put some pictures up later if my description is as bad as I think it is.

It actually looks okay as long as you don't look too closely, but I'll always know its there if I don't fix it, which bothers me.

My questions are:
How do I fix it? Just start over from a new black spray? I could also thin down some black paint and hope that doesn't leave brush lines...

What do I do next time I want to black wash a black tank? Just buy a bigger brush? Thin down the wash?

Thanks for your time!