If you are out visiting MINI of MoCo way there are two Flagship Car Washes (http://www.flagshipcarwash.com/) with 6 to 8 bays for self washing (open 24/7), with multiple setting pressure wands, industrial vacuums, and other things at each bay (besides the normal car wash building). They are set up pretty well with the vacuums outside (before) each wash bay to give you something to do if the bay is being used, then large wash bays, and enough pull-off slots outside each bay to let you chami and/or dry your Mini.
One (North Bethesda) is about a mile south of the MINI dealership just off Rockville Pike (355), behind the Bagel City on the east side of the Pike. The other (Rockville) is about a mile or two north of the dealership, just north of the Rockville Towne Center, also on the east side of the Pike just past the Giant, turn at the light at the Seven-Eleven. You need to use the token machine to exchange cash for tokens but you can get a good deal if you time the cycles right. All the bays at both locations have the multi-function power wand, a powered wheel and tire brush, and then a powered soap brush (I don't like using the soap brush since I think it has a lot of grit stuck in-between the bristles). I usually use the soap functions on the power wand instead of the soap brush. I have seen enough people bring their own bucket and sponge instead of using the soap brush.
I just used the North Bethesda location yesterday to power wash my Jeep Cherokee ("Jake") before my daughter takes it back to college. I have never had to wait for longer than one person in front of me to finish.
As for Arthur Treachers... there is still one in DC on Florida Avenue, NW but the Fairfax Circle one does sound good with the Citgo wash and Advance Auto close by. I'm always one for combining trips.