Yeah, SG is VERY durable, if the carwash compromised THAT stuff, it'd probably cause plenty of other damage too. Of course someone could botch up the mixtures or something...but I wouldn't worry about it.
Of course, we're not touching (oops, pun) on the whole "carwash induced scratches" issue. But Accumulatorette runs her A8 through (truly) touchless carwashes when she's travelling (she doesn't let them dry it)- it doesn't even take off the *carnauba* on her car, much less hurt anything.