IMO pre-made's clean and shiny looks is yanking on your OCD tendencies for cleanliness and newness. Weathering goes against the grain of the throw away society that has groomed your perceptions to this point. The only way out is to jump in and swim 
Try water based craft acrylics and/or powders that can be removed if you dont like it. Stick to plastics till you have confidence in your abilities; it washes easy without damage.
