If you just want to glue the crack together, and don't really care how it looks, then I've heard that Weldon 5 will do the job. Use a syringe with a fine needle to apply it to the crack.
If you want the repair to be almost invisible, and are willing to do a bit more work, you might be interested in this RV-List posting that I saved. It was posted by RV-6 builder Noel Drew, from South Africa.
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I can share your feelings on a canopy crack. My RV6 windscreen decided to
start a crack from the roll bar down some 6 inches that probably started
from somebody else resting too hard on the edge. This is an early 1/4 inch
windscreen that Van offered when the slider was first introduced and to the
best of my knowledge is no longer obtainable.
I was considering my options when a glider pilot looked at it and said
"weld" it like the glider owners do. I suppose there is always a first time
for everything so I started seeking advice.
I first located some liquid methyl methacrylate monomer that is the raw
material used in the manufacture of sheet plexiglas. It is not the glue
that is sometimes supplied nor is it solvent. It is called stabilised
monomer. Having stop drilled the crack, I cut a V shaped groove on the line
of the crack leaving a narrow gap at the bottom and about 3/16" wide at the
top. I located a thick needle for a hypodermic syringe and was able to lay
the viscous liquid monomer in the gap with tip of the needle submerged to
avoid bubbles. After somewhat of a learning curve I was able to generate a
reasonable weld.
The problems you will encounter will be bubbles trapped in the liquid, a
flow of liquid down the slope of the windscreen, the monomer going solid in
the syringe, needle, and on the surface of the jar it is supplied in. I
should mention that the monomer will evaporate and shrink in the groove if
you do not get it to set quickly by exposing it to ultraviolet radiation.
At different times I used the sun (very quick) and a crack testing lamp from
the nearby engine shop. Less than a minute in Durban's sun will have it too
stiff to go anywhere while it will remain liquid for hours in the
ultraviolet free workshop. After a few false starts that were rectified by
starting over with the Dremel, the result looked like a metal weld with a
ridge of plexiglas on both sides. Careful shaving and sanding gave me a
surface that polished up very well after it had set properly.
The end result is a line in my canopy that is smooth and transparent but it
deflects the light as is does not entirely match the colour or refractive
index of the original material. I am satisfied with the strength of the
join and now regard the repair as an honourable scar on my companion and a
satisfactory compromise to the huge task of replacing the windscreen.
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No matter what repair technique you use, you should try it on some scrap plexiglas first. If you don't have some scrap, get some from another builder. There are thousands of them around.
