Service Bulletin 13-12-12

Apparently, several flying aircraft have experienced electrical failure caused by water entering the avionics bay by seeping between the upper edge of the firewall and the underside of the fuselage skin.  This Service Bulletin was issued to correct this.

After putting the SB off for six months due to my dread of removing the Upper Forward Fuselage Skin (with all its machine screws), I hit a stopping point with the main build (space to work) and could delay no longer. The instructions call for applying Boelube as a release agent to the underside of Upper Forward Fuselage Skin, then applying a 1/16th-inch bead of Permatex 16BR Black Silicone Adhesive Sealant (#81158) to the opposing face of the Upper Forward Fuselage

Doubler (yellow arrow in pic at right).  After reading the opinions of those who had done this procedure already, I stuck with the Permatex but used O-Ring lubricant as a release agent.  I found this stuff on the aviation isle at WalMart near the pool and spa supplies.

It turned out to be impossible (for me, at least)  to make a nice, uniform 1/16th-inch bead as called for, so I instead made a more or less constant-thickness ribbon between the aft edge of the doubler and the nutplate holes.  I then re-installed the fuselage skin and tightened all the machine screws, the idea being to squeeze the silicone into a continuous, uniform water barrier before it sets up, while preserving the ability to remove the fuselage skin without affecting the water barrier.  I tested this with some scrap aluminum, and it seemed to work after curing for about a week.  No way to know what will happen after it has been cinched up for longer periods.  I'll definitely have to remove the skin at least one more time (and probably multiple times) to make the modifications required for the switch to a Garmin radio.