Willy: How about working up a Paint.net tutorial if you agree??? Or if one exists for Flight Sim painting, point us to it.
Layers ??
Bump maps ??

Educate us.
Keep in mind that I'm pretty much self-taught over going on 20 years of doing repaints without a clue how to actually do them. But I did pickup a lot of handy tricks along the way.
Here's a basic PhotoShop tutorial courtesy of Tom Gibson at California Classics. This was one of my early reads on the subject
http://www.calclassic.com/repaint.htm
I couldn't get my head around layers until a little over a year ago when Nigel explained to me it was like laying transparencies on a projector. By turning the separate layers on and off you remove them from the picture. But with a paint program that does layers, you can modify the layers however you want. Before Nigel told me this, I'd been "faking" the layered paints by using varying levels of opacity and brush effects. Once layers clicked in my pea brain, I was like "How could I be so dense?". Off and running since. And I can't thank Nigel enough for his pointing me in the right direction. Although he does use PhotoShop, but we all can't be perfect. j/k Nigel is a great guy and I can't say enough good things about him.
Using the Paint.net plug in for bump maps, I take my texture sheet and turn off all the layers except the panel lines and rivets layers. I've found it best for me to do lines and rivets in separate layers so I can alter one without effecting the other. Lines tend to show up better than rivets and I'll adjust them around to where they're about equal. Back to the bump maps. Once the only layers that are on are the lines and rivets, I'll do a new background at a RGB of 128/128/128 which is about as neutral as it it gets for the next steps. I'll adjust the brightness on the rivets to where they are pretty much pure white. Light colored goes high in a bump map. Panel lines will go dark as dark = low in the bump map. At this point I'll run it through the bumpmap tool. The output will have a blue background. I'll convert to dds and then try it in the sim. This first pass will be high highs and deep lows, but it give me an idea on where to take it next. From this point it's just adjusting the brightness on source layers. Darken the rivets until they are how you want them and lighten the lines. It's going to take some tries to get it where you want it.
Almost forgot, Paint.net calls the plug in "normal maps" or something like that. To find the tool, on the top horizontal menus it's , Effects/Stylize/Normal Maps Plus. Click on that and you'll get something like this. I just used the normal bmp for the F8F fuselage to get this. But it gives an idea on what it'll looks like
The Spartan source file I did for the fuselage (raw and un-adjusted): On this one it's not just the rivets that I made high spots. The window frames, rudder ribs and some access panels as well.
Finished bump map (well, not so finished, as the lines are too deep and the high spots need shaved down a bit.
The lines and rivets still need toned down on it. I ran into other issues with it that pretty much have me stopped on that repaint.
Now Specs, I know what's going on there somewhat, but how to explain it as I did a lot of googling on those to find out what's little I do know although I suspect quite a bit more.