Canopy Crack Repair
Repairing small cracks in sailplane canopies can be done with a few tools and a product named Acryfix, which is available from Tom Knauff at www.eglider.org. In addition to Acryfix you will also need a Dremel Tool with one or two attachments, and emery board and a kit of Micro-Mesh, also available from Tom Knauff. I usually use a very thin diamond coated steel cut-off wheel and a small pointed tip attachment, also diamond coated. The last tool needed is a fluorescent bulb in a small,easily positioned lamp.
Using Acryfix works well for the most common type of canopy crack, the small one or two inch crack that begins at a screw location on the canopy rail. You can repair larger cracks, but this process is better for smaller ones. I have repaired cracks over a foot in length but the end result is like a scar on the plexiglass and bends a lot of light. Still, it is better than a crack, and much cheaper than a new canopy.
Before repairing the crack, make sure the canopy is well padded and protected. I use open cell foam sheets as surface cushions and props to position and reposition the canopy during the repair.
To begin, the crack has to be stop drilled,. The next step is to cut completely through the crack with the thin cut-off wheel. Once there is a very thin cut through the canopy, it is time to V the cut out. Using a small pointed dremel attachment, V out one side of the canopy, so that midway through the canopy you will find the smallest width of the V to be no wider than the cut left with the cut-off wheel. Once this is done the canopy is repositioned and the other side has the V cut. The widest part of the V on either side should ideally be about 1/8 of an inch. After the V is carved into the plexiglass take some medium grit sandpaper or an emery board and lightly sand the surfaces of the V so that there are no ragged surfaces. Finish with a finer grit.
The next step is the actual pouring of the Acryfix into the crack. I usually use blue masking tape to mask the edges of the cracks so that no Acryfix is accidentally applied to the canopy outside of the crack area. After masking the crack, also mask the other side of the canopy so that the acryfix aplication will not run past the narrow middle part of the crack. In other words you are applying a dam. The day before the repair, set the Acryfix so that the application tip is pointing up. If there are any bubbles they should rise to the surface before you do your repair. It is best not to have bubbles in the repair. Carefully let a small amount of Acryfix run into the crack from one end to the other. The more volume that goes into the crack, the greater the chance for bubbles. After each application of Acryfix, place the fluorescent lamp close to the crack. This greatly speeds up the curing time. Be careful not to place the lamp too close to the curing Acryfix, as it will cause yellowing in the repaired area. One foot distance works well. After several applications you should have the new surface of the crack slightly above the existing canopy surface. The other side of the canopy can now be repaired the same way, after you remove the tape dam and mask the edges.
Once both sides of the crack are repaired, it is time to bring the new surfaces to the same level as the existing canopy surface. It is best to take your time using 320 grit wet/dry sandpaper on a small sanding block to get nearly to the ideal surface level and then switch to a finer grit paper, say 600, and then to 1000 or 1200. Be careful, the Acryfix surface is a bit softer than the original plexiglass.
Once the crack surfaces are the same as the existing surface, it is time to use the Micro-Mesh kit. Micro-Mesh is a series of cloth squares with abrasive crystals used to remove scratches and polish the repaired area. I use it like wet/dry sandpaper. When starting with the most abrasive sheet, keep the sanded area small and slowly increase the area as you go to the finer sheets. When you are finished with the Micro-Mesh the repair is finished.
Several considerations should be noted.
The smaller the crack the better. The larger the crack the more chance of bubbles forming.
The older the canopy, the more chance for complications. Although the crack can be repaired, the finished repair will not be as nice. It seems that repairs made on older canopies end up with very fine micro cracks along the edges, which I think is caused by the reaction of the Acryfix on old UV damaged plexiglass. My opinion is that even this result is better than a crack that has been stop drilled.