So I've been experimenting with doing a wash using acrylics without getting tide marks. This current method seems good. It's a mixture of walnut colored water based stain, liquitex slow-dri blending medium, matte medium and a small amount of water. The surface is moistened with clean water applied using a wide flat brush. Then a pointy water color brush is used to apply the wash in spots. Work in small patches and soak off or wipe the excess wash with a clean brush before it dries. It's a rather slow method though as the whole tank took almost 2 hours to complete the wash.
There's a massive sink hole at the rear axle. I'll cover it up with mud, hehe. :)
That painted vision slot is looking ugly, I'll tidy up with paint.