hey gb, sorry, i've been meaning to respond to this. we're getting ready for football, so work keeps getting in the way of my time here. the noive o' dem.
the combination of wood ash and water creates lye, which will accelerate the rusting process. ayrton, you must use very little water and remove any remaining ash or ashey paste to be successful with it. just one chink in the paint, and you've got bare metal, and then rust. add lye, and it'll rust thru in no time.
if you want to save your charcoal, you just have to remove the oxygen. close up all vents on the grill and cover it tightly. it'll smoke itself out. you can toss in a piece of newspaper to help it out. it'll burn down a little more, but there should be coals left.
i've been in bad conditions in the mountains where we've really needed to preserve firewood and charcoal for cooking, so we put out the fire when it wasn't needed by smothering it with sandy soil. the coals will continue to smolder for some time, so you can restart a fire easily if you do it right. also, it's a good place to sleep on top of when it realy cold. i guess if you're really into it, you could smother the coals in your grill with play sand, then pick out the useable pieces, and reserve the sand to use again.
imo, as i've mentioned before, real hardwood lump charcoal is the way to go for cooking, if you can afford and/or obtain it. there is no comparison for flavor, and different woods will impart different flavors. aslo, it should be started with a chimney, not lighter fluid. the latter is always fun, but i can always taste it in the food, no matter how little was used.
i will resort to using briquette charcoal if that's all that's available, but i prefer lump hardwood.
and you don't need a fancy or expensive charcoal grill. i have some cheapo $40 walmart special, and it's worked fine now for about 5 years or so, in all conditions. i'm just careful to be vigilant removing old ashes, and not letting any water get inside.
here's a picture of my no-name cheapo grill, cooking a coupla pork tenderloins on a cold night.
http://img213.imageshack.us/img213/7568/grillinginwinter4uy.jpg