How to get the cracked or patinaed look in gimp

Inside info comes at a certain price my friend.

How I do it (not really the best method).

I use someone else's texture. So if I am trying to get that distressed look, I Google search "grunge texture." There are some good ones on DeviantArt (note, as you are using someone else's work, even just for texture, you really can't use it for commercial purposes).

I add the grunge texture in a layer over the my image. Then I change the layer properties to either multiply, overlay, screen or additive depending on the color.

You can do it from scratch using add noise but I can't ever create the desired effect using that.