Steve, I use to believe big time in preshading. It works well for a single light color like under the plane but for any camo ontop you tend to loose the detail.
I have lately gone to post shading rather.
I use a fine soft pait-brush and use dark grey chalk-pastel that I shave off with a blade into a littl container.
hen I carefully "drybrush" over the panel lines.
I find I have way more control with this way.
Just be careful handling the model while doing this, you need to wipe off all finger marks before hand otherwise you get a nice fingerprint that can be lifted by the cops ;-)
You can do this only over a matt or semi matt finish for the powder to adhere to .After all I seal it in with my final clear coat.
This works well for exhaust and gun stains aswell.
And yes I too am an "RC'er"
Started with plastics as a small lad, did RC in High school, Went to the Air Force for national service, then started building and maitaining full size planes to pay the bills.
Now I havwe come full circle and am back with plastics LOL!
Theuns