Thanks Sean, I hadn't heard about the problems rubbing down over resin.
A buddy of mine has told me about the hairspray method where you put a silver base over your primer and leave to dry solid. Then spray with a coat of hairspray and leave that to dry. Finally spray with your black topcoat til it's dry enough to touch then go at the weathering detail with a short bristled brush (like a cut down artist's brush) to pull away the black.
He's showed me his scope weathered using this method and it looks really good. I was going to try it out on my scope and see how it faired before I went the whole hog and did the same on the blaster itself.
I'm still a little unsure about it, which is why I'm looking for different views at the moment. I'm ok at drybrushing, I used to do quite a lot back when I painted Warhammer stuff, but depending upon what's going to give me the best results I'm staying open minded.
For now at least.
****update**** The hairspray method failed for me, had to strip the scope down.