I had better results using plain "black acrylic wash"...
I took a tube of flat black acrylic paint. Squeezed about and inch into a spray bottle (clean "spritzer" type like you can pick up for a dollar at WalMart-Target, etc). Personally, I seem to have a better result with the "mix" if I use warm tap water...I am sure it is just me, but I think the paint tends to dissolve a little better in warm water. Anyway, to mix paint and water, I shake the "dickens" out of the spritzer bottle until it is all dissolved.
Apply to structure or scenery, etc. LIGHTLY. If it "runs" it looks funky. Multiple coats seems to let me decide when I put on enough (dry between coats).
I have been told that IF the structure has not been painted, it might be "too slick" if bare plastic, thus a good wash in soapy water, air dried, then followed by a quick paint job then weathering wash...OR...if factory painted, try the soapy water, air dry, then try the weather wash..."IF" still unhappy, go back to the soapy water, air dry, THEN try a flat finish clear coat prior to weathering wash.
Full disclosure statement: I don't do much weathering or painting these days. Most of my hands on was a few years ago doing scenery, but I have done a few buildings "repaint and weather" and a handful of rolling stock weathering.