Hello bids33,
For the weathering on my Tiger, I used Pastel Chalks (you can find these at any craft-type store) Do not use oils chalks. They don't sand down to a fine powder like the pastels do, they tend to get crayon like. As for the weathering powders, I have never used any of them myself.
I sand the pastel chalks until I have a pile of fine powder. I like to use, brown, tan and black. They can be rubbed on or "flicked" on (just dip a dry paintbrush in the powder and flick with your finger) I keep the flicked powder in place by giving the model a coat of Testors Dull Coat
first. This way, it adheres the pastels to the model. If you use dull afterward, the pastels tend to disappear.
An example of this is in my pic below. Where the panels were removed, I wanted to show some heavier mud & dirt build-up. I shot the area with the dull coat and immediately flicked the pastel on. Once dry, you can dry brush the area if a rusty look is wanted. Be careful to mask off the areas you don't want to weather. You want to keep the pastel in a certain spot.
Also, pastels can be easily wiped off (if not applied on top of dull)) For this reason, it is the very last step in weathering. You can use a brush, dipped in water to remove.
I would definately do a test on a spare kit if available, going slowly and adding little by little. This way, you would get an idea on how much to apply and what sort of effect you are going for.