I'm trying to paint a G-4 (had to use the void.bmp from the G-2) but the lines and rivets are WAY too visible. I have no idea how to "Fade" them so they're, at least, less visible. I'm using GIMP 2 for my image editing software. Any advice is greatly appreciated.

I haven't used GIMP but I believe it is a lot like PhotoShop. You should have the ability to read .psd files and use various layers. If so, download this template. It should have the G-4 within it.

 

What you'll do is deactivate the layers for versions that you don't use (Fs, G-6/10, Ks, etc.). Towards the bottom/base of the layers you'll want to do all your painting. The layers above will add the lines and rivets, and weathering. You can lighten and darken these as you want but the good thing is you don't have to draw them yourself.