Weathering Cars and Trucks

All depends on the kind of "weathering" you are attempting.

I use dry brushing with metallic colors to bring out the worn metal look and thinned washes of various dark shades to grunge up and bring out panel lines (washes should be of different paint type than the underlying paint; acrylic wash on enamel OR enamel wash on acrylic for example).

The weathering powders have their own merits as well and probably have as many different uses as there are users. I use the powders for exhaust and oil stains, touching up spots that just seem too pretty after the model is complete, etc . . .

As Tojo and Don say, practice, practice, practice. Start with a very tiny amount of product on the applicator and have at it. You'll see.

I generally put more weathering into my ships than into cars.  Some cars I do not weather at all, only racing cars appropriate for dirt tracks or rally stuff.  I weather almost all my ships.

Next to armor models car models are probably one of the most weathered type of model kit.

Lots of info out there from forums to videos to publications.

Google is your friend.

Having said that I have the Tamiya sets as well as powdered pigments. I personally prefer pigments, but that's just me. Nothing against the Tamiya sets.

I often use drybrushing for weathering both cars and planes (ships too).  It is not great for dust, as it leaves lines of demarcation which are a little too sharp.  With practice, however, you can soften these edges. It works great for scrapes (use silver or steel colors) and for exhaust stains.

Practice,practice,practice,also check YouTube for demos,so you can also SEE how it is done

Making rally cars look realistic and also making the new model (photo below) look dusty. I also have a few speedway cars that I want to make look muddy.

What exactly is it you are hoping to do?  What kind of weathering are you going for?  I own and have used several of the Tamiya weathering powders.  I personally think they are great.  But I use them on military jets so I'm not sure how to offer advise when it comes to autos.

I also just started using enamel washes.  Except I have the AK and Mig brands.  But I'm sure they work the same.  I have learned that using their own thinners is a good idea.  Hardware store thinners can be too aggressive on the underlying paint.  The underlying paint, BTW, should be acrylic (or something that enamel thinners won't ruin).