Okay Brett, I like to use these pastes in conjunction with pastels to weather my chassis/engine bays.
They are called Rub and Buff and are available at Michael's. I just rub a little bit on my hand and spread it around with my finger, you have to go easy, a little bit goes a long way. Just spread it all over your chassis and once done, use pastels for the dirt effect. I like to use a brown color, gives it a dirt/rust look.
Here is a '59 Caddy chassis I did recently, as you can see, I go easy with the pastels, just want to have a slightly used look.
Hosted on Fotki
Hosted on Fotki
Also did my Starliner in the same manner.
Hosted on Fotki
At the end, I will also use the Citadel washes throughout the mufflers, transmission and lighter colored mettalics like the pipes and other areas. You can spread your pastels liberally since they dont cover fast and you can always blow off any excess. As you can see, it's pretty easy and quick and gives the chassis a little more character and realism!!!