I'm with Bryan S. on this one, I never brush down the grill grates after a cook-up on my little Q120 and after almost 3 years,I haven't got one speck of rust on them. Mind you, the little Q doesn't get time to rust because i use it practically 5 nights a week all year round
I always give it 10-15 minutes heat up time before grilling then i give it a good scrub with a brass brush and it comes up good apart from the outside edges but they get done with the thorough clean up every 3 months.
I buy some non-toxic degreaser and give it a good hit all round, inside out & underneath, I take the lid off and wash the split pins seperately then scrape the inside of the base bowl with a plastic el-cheapo spatula and all the loose gunk will fall into the pan underneath, once clear of excess gunk, I then hit it with my high pressure water cleaner to get the degreaser and ground in gunk off and it comes up really good( don't forget to remove oil drip pan first otherwise you'll be looking like Al Jolson).
You can use a garden hose with a high power nozzle but a pressure machine is great.
I do however, have a hard time keeping the lid looking like new, in fact I find it impossible to clean all the black from the lid underside so i just clean it up the best i can. I find that the light colored lid stains easily and not easy to get off even with a high pressure water blaster.
I leave everything under the sun to dry and then tip the main body upside down for any water that might've seeped into the gas lines, put it together,hook it up and fire away..never has failed.I'll then put a good spray of oil on the grates and have never found a rust spot.
I'm pretty rough & ready but that's my quarter yearly clean-up and I was just thinking it's now overdue for the post summer(Australia) scrub so it looks like I'll be busy this weekend.
Cheers
Davo