Step 1:
Use an old toothbrush (or somethink like that) and lukewarm water with a little bit dish liquid to clean the model.
The support material for the "Detail" materials is some kind of wax. The color won't adhere if the surface isn't clean.
Setp 2:
For Shapeways "Detail" materials, use a primer.
spray vs. paint
Well, that depends on the model's size, and the kind of paintjob.
I prefer an application by brush, but for bigger one-color-paintjobs a spray makes sense.
I use waterbased acrylic colors.
Cheap, non-toxic, easy to handle...
Just give the model an additional coat of clear varnish after you paint it.
Other people (mostly railroad modelers) use enamel colors with great success.
I have even seen one guy who used a lacquer spray paint which was intended for cars.
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