You can use Tamiya paint for washes. Thin it with water and a couple of drops of dish detergent. THis will lower the surface tension of the water and allow it to spread evenly without it beading up.

It is best to apply it too thin then too heavy. By going with a thin wash you can gradually build up the wash color with mutiple coats, allowing it to dry in between coats. In your case I would apply a couple of washes of grey to lighten and take the shine off the model. Follow this with a very thin wash of hull red, or black.(Or mix them together) THis darker color will find its way to the recessed lines and pop the details out.

I few well applied washes can turn a plastic tank into a Model Tiger. If you stick with acrylic washes , and if you don't like the result you can spray Windex on the tank and with a soft cloth remove all the washes you applied, bringing you back to the base color.

You can go to great effort to weather a tank and many do(including myself) but this wash technique can give darn good results all on it's own in a short amount of time.