Depends what you mean by 'weathered'. Kind of tricky with a G36, since they tend to be used a lot by police and such, so they're not in the field for long and will look fairly pristine because of that. Having said that, the ones which do take a real battering, are the Kampfschwimmer ones (German Navy special forces divers). Those guys are trained to approach targets underwater, and come out of the sea firing their G36s and P8s, so their weapons certainly get exposed to a bit of wear and tear. The polymer furniture on them will of course not show much signs of wear, but what does show signs of wear is the three-tone camo which many of their G36s sport, so you could paint three tone camo and then buff it off the edges with a dremel or some such, however, there's something you could do which might be more fun that that. Take a look at this picture:
http://i.imgur.com/NreBfD4.jpg
You can see on that pic that the paint/bluing on the edges of the metal parts is very worn and showing the bare metal, i.e. the cocking handle, the RIS mount (particularly at the front), the flash hider, the fire selector, the body pins etc. Now, the problem you have, is that a lot of that stuff on an airsoft G36, is not metal, but ABS plastic, but you can still make it look like worn metal by using dry-brushing. If you've never done that, here's what that technique looks like:
That is my plastic UHC AKS74U, which I have dry-brushed so it looks like worn metal. So, how do we do that?
Right, what you want is some of this, which any decent craft or hobby shop will sell:
http://modelshop.co.uk/Shop/Item/Humbrol-metalcote-polished-aluminium/ITM3289
So, then what you do is, get a very soft artist's paintbrush (don't use a sh*t cheap one), shake the paint tin and then stir it with a match or cocktail stick so that the metal flakes are suspended in the paint, then dip the brush in the paint, then take a piece of kitchen roll and clean off the paint from the bristles until there is barely any left coming off the brush onto the paper towel. When it is like that, you are ready to use it - Lightly flick the brush over the edges and raised parts of your gun where you want it to look like paint or bluing has worn off. The almost dry brush should only deposit small amounts of paint on raised edges, and it will look like worn metal if you do it right. Don't overdo it, subtlety is the key here, and remember that much of the real G36 is not metal, so you might need to put masking tape on the bits you don't want the paint to go on (crappy 'pound shops' will sell masking tape). If you need to mask around intricate parts, Blu-Tack is good for that, because you can mold it around things.
Now, leave it to dry for a couple of hours, then you need some aerosol matt varnish (you can get that from most 'pound shops' too). Spray that very lightly (and I mean VERY lightly) over your bry-brushed bits, and it will prevent it from rubbing off. Be careful though, the wet varnish will tend to wet the metal paint and spread it out, which is not what you want, so if you don't work slowly with the varnish, it will ruin the effect, so, the idea is to do ONE very light quick pass with the varnish, then let that dry, then do another light quick pass, and so on. You need to work slow with the varnish, and let each pass dry before you go again, or it will ruin the effect. Patience is the key word here.
