Here's a better idea - try dry brushing on either a junky old model, or on scrap plastic, paper, or wood. Use different surface types to get an idea of how to apply paint instead of simply using chalk powder which doesn't stay in place, isn't effective, or realistic, and unecessary. Look at real world machinery, and cars with huge rust, and weathering stains. Go to your local junk yard, and take pictures for reference. This will give you an idea of how to recreate this look. With dry brushing you can also use mineral spirits to thin down your paint, then apply as needed in areas where most people who aren't familiar with this technique in crevices where it much harder to get a pin wash to look right. This allows for a more controlled application, and is lot more realistic than any powder. Use old T-shirts and cotton swabs work real well for blast marks, and soot stains. This is what I call the ILM weathering effect as this is what they did and everyone else did back in the day since there were no chalks, or any reason to use them! It's all in what you know, and what you have to use as well as how to use it to the best of your benefit.
~ Cobra Chris