When building my 235 wing tanks, I was concerned that the drain valve wasn't located at the lowest part of the tank, possibly allowing water to accumulate next to the BL-51(?) rib. Here is what the instruction manual showed:



I chose to mount the flush drain exactly in the lowest portion of the bottom wing skin where there was no foam core. I drilled a hole at the lowest point then used structural adhesive to glue the aluminum hard point onto the outside of the bottom wing skin so that the drain holes in the quick-drain were peeking just above the interior of the tank sealant. Afterwords, I fiberglassed over the hard point, which I had previously rounded and beveled. When it came time to finish the aircraft, I used micro to build up an aerodynamic "blister" to cover the hard point and quick drain. I probably lost a couple of Potential Knots of airspeed by sticking the drain out into the wind, but I won't have to worry so much about un-drainable water in the wing tanks.

Speaking of water, we had a recent weather change here in NC from 40-ish to warm and humid and rainy. The entire aircraft was sweating in the hangar - not just the engine but the fiberglass airframe as well. I'd never seen anything like that and I have to assume that under such conditions, moisture could also build up inside the fuel tanks. It's something to think about. Here's what my "blister" looks like, and you can actually see the condensation in the photo: