You have to put the chalk/powder on a little heavier than you want it to look. When you spray the dull-cote at the end..... YOU DONT NEED MUCH.... a very little is all it takes. If you are using the spray can, hold it a long ways a way from the truck. You just want a very fine mist.....or it makes all the weathering disappear.
I use the bottle of dull-cote thinned 50/50 with laquer thinner and spray it with my airbrush at about 10 psi. Just a little.....is all you need. 
You can also brush on some chalk/powder while the dull cote is still wet. Especially with light colors like grey or white. In the 1st truck picture I posted, that light grey on the sides of the truck were brushed on while the dull cote was wet. Spray, brush, spray, brush until you get the effect you want. Takes practice and patience..... and I'm the last person on the planet to preach patience 
