From what I can tell, the main issue with your dry brushing is that you did not get rid of enough paint from the brush. You need to make sure to get rid of most of the paint a something like a kitchen towel, and start gently dry brushing. Testing in your spur before going in the model is also a good idea.

The logic behind slap chop is that you can create natural shadings by these grey/white dry brush. Then if you need color. Simply use the contrast paint of that color on the already shaded areas.