Weathering is caused by things spraying up onto the car and drifting down on the car, then the actions of the rain on that dirt.
Chalks only replicate some of those actions.
Washes are very useful.
You also need different colors of chalk. Black chalk would be more useful than red. Try and find some earth tone chalks, white, tan, orange, brown, black.
For wood cars I like colored pencils. I bought a couple shades of grey (not all 50) and at a craft store found a pencil set designed for drawing people of color, having a variety of browns and tans in it. On the wood cars I streak along the boardswith colors similar to the car color then with some grays and browns to represent areas where the paint wore off exposing raw wood.