Page 25A-04 Bonding the rear Plexiglas window to the aluminum

Back in September of the miserable year that was 2020 (and now that I've seen the entirety of 2021 I think it was worse) I described the expensive and tedious process of fitting and drilling the rear window. It was expensive because I had to buy another window.  The last task in the window installation was to bond using ProSeal (my 431st favorite substance) the PlexiGlas to the aluminum turtle deck.  In order to allow easier access, I put this permanent attachment off until everything aft of the seat backs

(ADAHRS, ELT and fuel tank) had been installed.  The forward edge of the window screws into the previously drilled and tapped holes in the roll bar, and this attachment is done first.  The aft and side portions of the window attach with screws and nuts to the aluminum turtle deck through previously drilled holes.  All this is done before bonding with all fasteners loosely attached.

ProSeal has a six month shelf life and I had bought this batch to seal the fuel tank back before the move from NC approximately two years ago.  I would definitely not use expired stuff on the tank, but for this application I simply mixed up and observed a test batch to make sure that it would set up.  I'm never confident with this stuff.  It has to be mixed 10:1 by weight, ProSeal to catalyzer using a postal scale.  It took longer to set up, partly owing to low temperatures, but worked.

As shown in the first picture, I used ice cream sticks to create a space for the ProSeal between the aluminum and the PlexiGlas.  A gob of catalyzed ProSeal about the volume of two golf balls was put into a zip lock baggie, a corner was cut off the baggie about 1/16th inch, and I squeezed the stuff into the gap as if I were decorating a cake, removing spacers as I went.  All fasteners were then given their final low torque and the squeezed-out ProSeal was scraped up using the rounded end on the smallest ice cream stick.  These words don't come close to conveying how messy this process was.  If ProSeal gets onto any fabric, it's there forever.  I have a particular pair of pants, shirt, socks, etc that I wear for any operation involving ProSeal.

A close inspection of the first picture will reveal tape following the contour of the aluminum and tape spaced 1/16th inch from the aluminum on the PlexiGlas.  This, in theory,  prevents ProSeal from getting on anything it shouldn't be on.  The problem with this is that cutouts must be made in the tape on the aluminum around every screw head, leaving little room for error.  Acetone can be used to clean up errant ProSeal from aluminum.  The ProSeal simply can't be allowed onto the Plexi outside the 1/16th inch strip.  This process was not fun and I was not pleased with the result.

The real problem, or so I thought, was that I'd foolishly applied the tape to the Plexi back in NC two years ago.  As a result, when I attempted to remove the tape the adhesive remained on the Plexi while the tape backing peeled off.  A search ensued for a chemical which would remove the adhesive while not damaging the Plexi.  Opinions on the forums and internet in general were all over the map, often contradicting one another.  With fear and trepidation I settled on naphtha and applied it to the offending adhesive.  Worked like a charm.

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