The last step is to add a little more weathering with colored chalks. All you need is enough to bring out some more of the details in the wheels and add a splash of color.
By now you should be able to remove the load from the car and peel back the plastic bag. It is easiest to do this before the glue is completely set. Let the glue finish hardening with the bag removed before putting the load back in the car.
Your load of wheels is ready to go! As you convert more cars, you can make a few more. Now if there was just something you could do with those old horn-hook couplers!