Chapter 25: Window Edge Treatments
(Not addressed in the plans.)
One of the more challenging cosmetic issues is getting that professional-looking seam between the windows and the surrounding fiberglass surface. Because of the way we install our windows, it is difficult to get all of the window edges to cure firmly against the turtleback's outer skin. The problem is exacerbated by the fact that the suppliers' pre-made windows rarely match the curvature of our self-made turtleback. Thus, the window often cures a slight distance away from the fiberglass, with the gap being taken up with flox. These mismatches usually occur in the corners.
Since the plans don't address how to deal with this problem, I posted a question to the Cozy and Canard Aviators groups, "What's the best technique for cosmetically treating the interface between the outside of the window and the adjoining, external fiberglass? Bead of micro? Silicone? What?" The overwhelming answer was to use dry micro and apply a bead of it around the window. Once cured, carefully chamfer the edge. A few people used a rubber-like sealant compound actually made for sealing automobile windows into their frames. A few people just covered up the gap with trim tape.
I decided to go with the dry micro bead. The process I'm using is described below.
For Step 1, I trued up the perimeter of the window framing with a razor blade and a Permagrit file held on edge. I did this to remove any excess flox and more importantly, to remove any electrical tape or plastic that was previously there. You have to be VERY CAREFUL while doing this. You don't want the razor blade to slip on you and scratch/gouge any other part of the acrylic. I wiped the window edges down with some Goof-Off to remove any glue residue left over from the previous tapes and plastics. My windows are fitted without large mismatches. So for Step 2, I decided that a 1/8th-inch edge was all I needed cosmetically. So I bought some 1/8th-inch masking tape from my local auto-body shop and taped it along the window's perimeter. The masking tape is used only as a reference so that you have a guide for applying the boundary tapes. For Step 3, I butted some layers of electrical tape against the masking tape to form the boundary for the micro. The picture on the left shows the starboard side window on the canopy windows with the 1/8th-inch reference tape (the blue stuff) and the electrical tape butted against it. I did the same thing for the canopy bubble, except that I chose to use a 1/4th-inch chamfer to help minimize that damned depression (cyclops) between the canopy bubble and the turtleback.
With the electrical tape in place, I removed the masking tape and CAREFULLY scuffed up the exposed acrylic. (Step 4.) I vacuumed away any dust and debris, then wiped down the exposed acrylic with a clean rag.
I then applied the bead around the perimeter of the window. I said "dry micro" earlier, but after giving it some more thought, I used the WEST 410 microlight filler instead. It is more creamy than dry micro, it spreads easier, and it sands easier when cured. I just smooshed the mixture in place, then removed the excess with the squeegee. (Step 5.) As soon as possible, while the microlight was still nice and creamy, I CAREFULLY lifted the electrical tape off the windows at a 90-degree angle to the microlight bead. (Step 6.) This produces a very sharp edge to the filler left on the window! This picture is a bit deceiving. The black electrical tapes you see in the picture are actually on the inside of the window. There is no tape on the outside of the window. As you can see, pulling off the border tapes left a very sharp edge on the microlight. I was pleased!
After curing overnight, I reapplied the electrical tape in layers against the microlight edge. I CAREFULLY sanded away the excess microlight. Believe it or not, I still used the tried and true 45-degree sanding pattern!! Look closely at the picture and you can see the scuff marks on the boundary tapes. The boundary is only one layer at this point. So I had to be extremely careful with the sanding. The other picture is of the final product! I am way pleased with how these edges turned out. The next steps are to skim coat the canopy, ensuring that I skim coat the microlight chamfered edges. The raw epoxy will soak into the chamfered edges and strengthen them against chipping.
