Weldon 16 is cracking my canopy. Why???

[dwranda]

I had a crack in my canopy and decided to fix it as well as I could using the weldon products. I drilled a hole using a plexi bit to stop the crack. I filled in the crack with the thinner weldon #3 and sanded it down so its smooth. I then filled the stop drill hole with #16. I checked it about an hour later and there were little spiderweb cracks emanating from the hole. I had no idea how they showed up. I drilled those out and just filled those holes with the 16 again. 10 minutes later more cracks showed up. I drilled a bunch of practice holes in some scrap and can't get any cracks to show up in the scrap pieces. Why is my canopy cracking when I put the weldon in the hole?

[lr172]

Not sure, but my guess is that the weldon is expanding as it drys / cures. Try filling the hole with epoxy or clear silicone .

[rocketbob]

Because of the curvature internal stresses are high so its likely because of the holes you've stop drilled. I'm of the opinion that stop drilling cracks usually makes a crack worse. All it does in truth is shows others that someone has acknowledged a crack. I can't tell you how many times I've seen aircraft skins stop drilled with cracks emanating from stop drilled holes.

[Captain Avgas]

Weldon 16 is recommended for acrylic products but in fact it is a solvent based product containing a high percentage of MEK. It works by actually dissolving the acrylic. The problem here is that MEK is a double edged sword as it can break down the molecular structure of the acrylic and potentially cause additional stress cracking as you have found. The problem with the Vans canopies is that they are double curvature and the forming process leads to increased levels of internal stresses being locked in. Trying to repair cracks in Vans canopies can therefore be very problematic and is best done by a professional.
My guess is that you are going to end up replacing this canopy or you are going to have to live with something that looks terrible.
Many builders think that using the Sikaflex approach on their canopies will eliminate the problem of cracking but there is mounting evidence that this is not so.

[mbauer]

Used to work in a plastic shop in the 1980's.

Do you have any scrap pieces of the canopy? If you do; you can use a rasp to create little flakes, dissolve them in the Weldon and use as a filler. Thinking it used to work with the #3.

Never had any issues filling gaps or holes that way, thickens up as you add the flakes.

Only fill in a little at a time in thin layers if plugging a hole. Once done sand smooth with super fine sandpaper to get it to almost disappear.

Best regards,
Mike Bauer

[RV7A Flyer]

Originally Posted by Captain Avgas

Many builders think that using the Sikaflex approach on their canopies will eliminate the problem of cracking but there is mounting evidence that this is not so.

Can you provide more details here? I'm interested in hearing about it...

[Captain Avgas]

Originally Posted by RV7A Flyer

Can you provide more details here? I'm interested in hearing about it...

Just use the search function. There are plenty of threads that detail cracks in Sikaflexed canopies. And this thread is just another one.

[jcarne]

Originally Posted by Captain Avgas

Just use the search function. There are plenty of threads that detail cracks in Sikaflexed canopies. And this thread is just another one.

While you are correct that there are still cracks when using Sika wasn't there a poll not that long ago that showed the percentage of cracks was still less? My personal theory is that if you keep the primer off of the edge of the canopy you stand a better chance but hey that's just my theory.

[Captain Avgas]

Originally Posted by jcarne

While you are correct that there are still cracks when using Sika wasn't there a poll not that long ago that showed the percentage of cracks was still less? My personal theory is that if you keep the primer off of the edge of the canopy you stand a better chance but hey that's just my theory.

This is the most recent Sikaflex poll that I am aware of. The failure rate for Sikaflex canopies turned out to be amazingly similar to the failure rate for canopies attached with fasteners. This, of course, was not what the early adopters of Sikaflex were expecting. Cracking aside, canopies attached with fasteners have a 100% success rate over a 40 year period in terms of structural security while the jury is still out on canopies attached with Sikaflex.

http://www.vansairforce.com/communit...=Sikaflex+poll

Incidentally, I think your theory about keeping the Sikaflex primer off the canopy edges is logical. The primer contains a lot of really nasty solvents.
In the final analysis however I think that what we are discovering is that cracking is the result of poor canopy construction techniques....regardless of how the canopy is attached. You can get away with a lot of terrible workmanship and riveting on an RV but the canopy is very unforgiving of poor fabrication techniques and impatience. I personally found it to be the most difficult and most demanding part of the build.

[jcarne]

Originally Posted by Captain Avgas

This is the most recent Sikaflex poll that I am aware of. The failure rate for Sikaflex canopies turned out to be amazingly similar to the failure rate for canopies attached with fasteners. This, of course, was not what the early adopters of Sikaflex were expecting. Cracking aside, canopies attached with fasteners have a 100% success rate over a 40 year period in terms of structural security while the jury is still out on canopies attached with Sikaflex.

http://www.vansairforce.com/communit...=Sikaflex+poll

Incidentally, I think your theory about keeping the Sikaflex primer off the canopy edges is logical. The primer contains a lot of really nasty solvents.
In the final analysis however I think that what we are discovering is that cracking is the result of poor canopy construction techniques....regardless of how the canopy is attached. You can get away with a lot of terrible workmanship and riveting on an RV but the canopy is very unforgiving of poor fabrication techniques and impatience. I personally found it to be the most difficult and most demanding part of the build.

Ya that's more similar than I thought. Although I am curious as to the percentage of type out there (I would think there are A LOT more riveted than sikad at this point which the poll doesn't suggest, although I could be wrong here). As far as I know the Sika method is 100% as well if it was properly done but I haven't been around long enough to know this for sure. I totally agree with you on constructing the canopy, without out question the hardest part of my build too. I keep it indoors in a locked room in my house because I'm so paranoid of someone hitting it or doing some form of idiocy around it. I think I have about 160 hours into it and I still have to attach the windscreen.