Have a go on a scrapper, and see how it goes. I to would be very careful about trying something like that for the first time under a SIG build.
Have a try with the salt method
1. spray metal colour. leave to dry.
2, spray with water, sprinkle salt (I find the course best as you get loads of different size salt grains), where you want the bare metal to show. Leave to dry. Some will fall off.
3. Prime let it dry. repeat step 2 for where you want wear to show the primer.
4, paint the final camouflage. Once all is nicely dry, scrub off the salt. Some will be a bit stubborn and will need a bit of water to loosen it.
Should you wish to do the photo you posted, I would say the best way to continue, would be to spray the kit with hairspray, then paint the final camouflage. When that has dried, you can wet it with water. Loads of water, as it will need to get through the paint. This will make the hairspray dissolve and the paint can be rubbed off, some times you can even get it to peel.... ;-/
Ian M