I think I've tried every method/technique going over the years Dan. But I generally stick with the dry-brushing technique that Matt described. The only difference is, I very rarely, if ever, use straight 'silver'. If you look at paint chips or paint flaking on the 'real thing', you'll notice it is very rarely that bright; normally, the only time it is, is if the metal has been freshly torn up or punctured, either by accident damage, or bullet/shell holes etc. Then, it IS bright, as it's brand new, untouched metal. I normally add a touch of matt white, varying the amount to vary the shade,density/brightness. With practice, the dry-brush technique works very well indeed. It can be varied, by gently painting, in thinned colours, to achive the desired effect. However, as the area you are concerned with is the opposit, i.e. a large area of bare metal, with worn, scuffed, flaked and chipped paint remaining, I would do it the opposite way around. Paint the canopy section in the metal colour first, preferably with your airbrush, then give it a bit of a polish when dry. Now, you can either use your pull-off method, with tape, or use a masking fluid, spotted on, then paint the whole area in the cammo colour. When dry, pull off the spots of dried masking fluid. If you can't get this fluid (one type is humbrol 'Maskol'), use PVA adhesive, ever so slightly thinned with water. Alternatively, use the dry-brush technique over the 'metal' with a slightly thinner, perhaps lighter shade of the cammo colour. These all work very well indeed, and it's only the amount of brushing/masking etc, plus practice, that will affect the result. As Matt said, try it out first, on a piece of scrap plastic, an old model or something similar. I'll see if I can get a close-up shot of the very weathered, and part paint-stripped Marauder I did recently, to show you what it can look like.