painted 3D printed items before they have all been printed from a high detail acrylic polymer, whereas the wagon is made using an entirely different process (it involves lasers). Fortunately, the answer is that the cheaper materials paint fairly well as the photo shows.
In the past I've painted entire wagons using aerosol cans of primer and paint. While this does work, the problem I've found is that while the primer goes on well the paint tends to go on quite thick and you run the risk of obscuring some of the finer detail. So on this occasion I used an aerosol of primer, but then applied the paint (Tamiya Color XF-1, Flat Black) by hand using a brush. This seemed to work really well, although it did need a couple of coats to get a good covering of paint. I then did a little dry-brushing (with Tamiya Color X-10, Gun Metal) to pick out the metal work and bolts. This wasn't a complete success, but wasn't disastrous either, and has helped to show off some of the finer detail in the model.
The main difference between this wagon and the others I've painted in the past is that the surface of the plastic isn't smooth -- I've seen it referred to as "sandpaper like". Now if I was trying to model a smooth surface (like metal) then this might be an issue, but the wagon is predominately made from wood, and even newly painted wood isn't entirely smooth. Personally I think the rough surface shouldn't be a problem, it definitely looks different but I don't think this is really a bad thing. The only issue will be how well waterslide transfers will stick to the surface. As the prototype wagon isn't really the right size (it should be taller) and is missing a lot of details I'm not going to waste a set of transfers on it, so that test will have to wait until I do another print run of the more detailed model.