I added chalk pastels in rust shades to the muffler to create the crusty rust seen in pics. Ordinary tap water held the powder together. The frame also received a few streaks of this mix, but kept light.
The photo's don't show the dust buildup on the box very well, but it was airbrushed w Tamiya deck tan and buff along the box bottom and where dirt would spray up from traveling.
Last thing I did was the warning sign on the back, a quick image search of the sign and printed off. I added this for a little color..
I was advised to add a light load to the box, since the drop axle was down. Blue foam made a fake floor, then cheap cat littler topped off the load..