Barrs makes a nice weathering powder,l too. I use this to create a "lightly" weathered effect. The packet comes with four colors, with black and three rusty colors. They also include directions which call for practice on a not particularily valuable item.
They don't necessarily call for the dullcoat. In fact, with Barrs, I just apply with a small brush (rubbing in with the bristles, using a dabbing and circular motion) and gently puff off any excess with a breath of air. Rubbing with your hand may leave a small residue on your hand. Some plastics or resins take the powder better than others.
When I tried to seal them with dullcoat, they seemed to loose their effect.
Being a relative beginner, compared to others here, I use only the Barrs, Alcohol and India ink, and watered down hobby paint as my weathering compounds.
These are messy like Motleys products, and require a clear work area and newspaper, no kids around.
Here is an inexpensive locomotive that I may have gone a bit overboard on:
Hope this helps!!! lasm