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Lord Adhesive vs. Weld-On Update

Brandi

Well Known Member
I don't know if you guys remember, but we attached the pilot side door window on our plane with Weld-On and the other windows with Lord Adhesive as a little experiment. Some builders feel that the Weld-On is responsible for the small cracks that form around the window over time. Personally, I was sold on ANY alternative after working with Weld-On for the first time; what a huge mess! Note that, for all windows, we left the appropriate (and same) size gap around them and then filled in this gap with filler as instructed in the plans.

Flash back to OSH 2012... at around 55 hours, a friend pointed out a "ridge" forming around our Weld-On window. It was pretty nasty looking and would have definitely caused some paint issues if the plane would have been painted already. Since then, this "ridge" has covered more ground around the window but has not really changed in height. Here's a photo of the Weld-On "ridge" when we first discovered it.



Yesterday, our painter, Don, pointed out some cracks around the rear windows and doors in which we had used the Lord Adhesive! Granted, they are NOWHERE near the size of the strange "ridge" that formed on the Weld-On window but, they're still there. We're now at about 270 hours and I've never noticed these cracks before so I don't know how long they've been there. They're pretty small so it isn't something that you'd see on an unpainted plane unless you were, say, prepping the cabin cover for paint. It might not even be something that would have shown up through paint already; I have no experience in this area. Below are photos of the cracks found in the Lord Adhesive rear and door windows, respectively. You can click them to open the full size image and get a better view of the cracks.





I find it very interesting that the cracked areas don't always occur at the edge of the window. A few spots are actually at the cabin cover, on the other side of the gap.

I still think Lord Adhesive is the better option but, it's not the ultimate solution. I'm not sure there is one short of masking the cracks by putting a few layers of glass around the seams. It seems cracks are inevitable when you have two different materials expanding and contracting at different rates. Anyways, just thought I would share more findings on our experiment.
 
Weld On 10

I used Weld On 10 to glue all my windows in place and after 440 hours of flight time I have zero evidence of any cracking or ridges or any kind. now I hasten to add that I did apply a 1" strip of 3 Oz. (I believe) FG cloth all around each window and feathered it in with epoxy filler.
The gap between the edge of the glass and the joggle in the FG top was not filled with Weld On but rather with flox and then the cloth strip on top of that.
 
We used the thicker Weld-On 45, 410 filler, light fg cloth. Ridging/cracking showed up after first hot summer. If it gets too bad, black trim paint may get applied.
 
I used Sika adhesive and this summer I have seen cracks show up in the corners of the rear windows. I used the same method as Ivan, with 3 oz cloth all the way around each window. If I were to build another -10 I would fix this by installing the windows from the inside. It would take a little work to modify the window frames, but in the end would yield a better looking installation and would allow for re-installation/replacement of a window without having to paint the entire airframe or door! IMHO the engineering of the fiberglass structure took second seat in the design process, especially when you consider the door attachment and structure.
 
I also used sika flex on my windows followed up with fiberglass around the windows. No cracks yet. I really think the fiberglass is the main reason I have not had cracks.
 
I used Sika adhesive and this summer I have seen cracks show up in the corners of the rear windows. I used the same method as Ivan, with 3 oz cloth all the way around each window. If I were to build another -10 I would fix this by installing the windows from the inside. It would take a little work to modify the window frames, but in the end would yield a better looking installation and would allow for re-installation/replacement of a window without having to paint the entire airframe or door! IMHO the engineering of the fiberglass structure took second seat in the design process, especially when you consider the door attachment and structure.

I love the idea of the window being fixed from the inside - a much better engineering solution. The only issue I can see is having to feather the flange to get a reasonable edge height. Couldn't agree more about the fiberglass quality - a real shame when the rest of the kit is so good.

I have used the Weld On 10 and laid up glass over the join. Even so, I am resigned to having cracks in the future.
 
I have used the Weld On 10 and laid up glass over the join. Even so, I am resigned to having cracks in the future.

Now you are in the right frame of mind. Cracks are okay. I don't see them when I am flying over the beautiful clouds and landscapes. I don't pay any attention to them anymore.
 
If no fiberglass is used over the joint it will crack no matter what it is glued with.
As Ivan said you need at least a total of 3 oz of cloth minimum. Also very important is to fill the gap between the window and the fiberglass top with epoxy and flox or fiberglass chopped glass as the filler between the two. The other
thing to watch for is if you fiberglass over the window joint you must not sand back thru it when blending it in to the cabin top. If you sand partially thru the let's say 4 oz cloth it will probably crack around the joint sometime.
The big advantage of the Lord adhesive is that it is a little stronger bond than the Weldon and much easier to use. It cleans up easier and is much more forgiving and a longer work time.

Geoff
 
Now you are in the right frame of mind. Cracks are okay. I don't see them when I am flying over the beautiful clouds and landscapes. I don't pay any attention to them anymore.

I'm with Wayne also. The cracks look much better unjust the paint than through cloth, so if laying up over the edge, make sure to do it well. Installing from the inside gives a much less smooth exterior, which would be a shame on such a smooth plane. Still quite possibly cracks would appear with different expansion coefficients.

We have used epoxy resin with micro to fill the gap with quite good results till some hairline cracks, but that's kind of the nature of the beast.
 
Topic Change - Interior installation of Windows

I used Sika adhesive and this summer I have seen cracks show up in the corners of the rear windows. I used the same method as Ivan, with 3 oz cloth all the way around each window. If I were to build another -10 I would fix this by installing the windows from the inside. It would take a little work to modify the window frames, but in the end would yield a better looking installation and would allow for re-installation/replacement of a window without having to paint the entire airframe or door! IMHO the engineering of the fiberglass structure took second seat in the design process, especially when you consider the door attachment and structure.

I did a briedf search for installing the RV10 windows with this method and did not find examples or a "how to".
Has this approach actually been performed? Or is this still in the theoretical stage?
Thanks,
Bill
 
I don't know of anyone who has done this. I doubt it would be any more difficult than the current method. There would be a small edge around the window, but so what if it fixes the cracking issue. I noticed at OSH last year that the Vans aircraft has cracks around the windows. The sika adhesive was designed for this application.
 
I will take a look

I will take a look and see what it will take. I think it will require reversing the flange, and coming up with a device to push/hold the window in place from the inside while the door is closed.

Will be interesting.
 
Want to learn more.

I don't understand how mounting the windows from the inside, against an exterior flange would solve the problem of cracking. Wouldn't it just move the cracks to the inside? Also, if expansion/contraction is the culprit, there would still be cracks appearing on the outside flange to window interface, wouldn't there? I'm nowhere near the window install stage, but I'm interested in learning from those who have gone before...
 
I used Lord adhesive and two layers of fiberglass tape over the seam. Although, I'm not quite flying yet, it has been ten months since painted and over a year since installed. No evidence of cracking yet. I was very careful not to sand into the glass when finishing.
 
window cracks

From my experience, small cracks turn into larger cracks. I?m in the camp of a removable window from the inside.:)
 
By mounting the window on the inside, you no longer have to paint the window. So there is no paint to crack! The cracking results from the dissimilar expansion rates. Bob, wait till it sits in the 100 degree sun a few times. I hope you are correct. I know Geoff used the lord adhesive and also has a small crack. The best part of mounting the window from the inside is the ease of replacement. I will admit, the sika adhesive is meant to expand and contract. That's why it works so well with bonding of materials with dissimilar coefficient of expansion. I initially did not intend to paint onto the window, but to create a pro seal bead around the edge of the window, just like the Cirrus guys do. I changed my mind, which was my mistake. I have considered fabricating a new door just to see how it would work.
 
Inside mounting of windows

Mounting the window inside and making it appear "seamless" on the outside while providing a robust attachment is going to be interesting. Currently envisioning the current window flange thickness sitting flush on the outside of the door, which means the window will be recessed by that anount. Could try match tapering both surfaces. It would minimally reduce the overall viewport size but would require some dedicated fixtures (male mold for window flange and taper transfer fixture for the window) and tooling.

Other ideas?

And if the cracks are non structural - is it worth the effort?
 
Bob, wait till it sits in the 100 degree sun a few times. I hope you are correct. I know Geoff used the lord adhesive and also has a small crack.


It already has. We had a few of them this past summer. I've got my fingers crossed that since I've gone through four seasons already, that I may be lucky and not have any cracks.

I seem to recall that Geoff mention that he thought he may have sanded into the glass covering the crack. I also used a heavier cloth than some have used.
 
Rudi Greyling used Sika to attach his windows. See this thread:
http://www.avcom.co.za/phpBB3/viewt...5d0b4c90e3eb0aef99f98888ecd&start=255#p754392

Also, he sent me these additional details via PM:

The 10 is more difficult than a 7 cause of the side windows thickness almost exact fit with little Sika bond thickness.

So you have to get the minimum thickness around the edge. Cut your windows 3 mm smaller than the recess in the fuse all round, and bevel the window edge 45 degrees inward. smear Sika and stick it on with clecos. Most of the Sika will peal out, but make sure you have 3mm edge. The bevel and 3mm edge bond with the thin layer between is enough to hold the windows in place.

Fill the cleco holes with Sika.
 
What I would do is to reduce the outer half door window opening by the amount of overlap needed for the glue joint. Fill in the current joggle with fg lay ups. This will leave the door skin bond flange around the window intact and not reduce the bond surface area. This will also leave an inside joggle in which to attach the window. I would use the Sika product for the adhesive. I would finish the edge of the glass by rounding the inside edge and finishing with a Sika filet. The Sika product uses an activator applied to both sides of the bonded surfaces that leaves the oposite side of the glass with a nice shiney black mask reveal.
The rear windows would be even easier.

I want to add that I did pull tests using the Sika adhesive. The substrate broke apart and the joint never failed In any of the test panels I created during the bonding process.
 
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I do have a small crack about 1.0" on the top of the wind shield. And it only cracked because I did sand almost all the way thru the glass cloth. I took the chance and decided not to repair it with fiberglass. It was more of a test than anything. You do not see it unless you are up on the wing. That being said if you sand thru the cloth or do not use enough glass or go without it will crack.

Geoff
 
I want to add that I did pull tests using the Sika adhesive. The substrate broke apart and the joint never failed In any of the test panels I created during the bonding process.

To date I know of ONE Sika glue failure of an RV-6A tip-up canopy; the builder did NOT use the Sika 209N Plexi Primer; did not even scuff the Plexi. He glued the shiny Plexi to the structure and the glue joint failed on the shiny surface. Hardly surprising. :confused:

Remember the article in Sport Aviation, 'Why do we still drill holes in Plexiglas?' I wrote that article and am glad to see that Sika has caught on with others. Initially I did not think this was a good idea in an RV-10, but now considering the problems being experienced by others, perhaps it is.

Last year, I bought an RV-10 from a friend's estate, the aircraft had 25 hours on it and had stood in the blazing sun in Botswana (semi-desert) for about a year. All windows showed signs of cracks in the paint and filler around the edges of the Plexi. The windows had been glued in with Sika 295UV. I laid up 5 layers of 160g fg cloth cut on a 45degree bias around the edges of the Plexi, to tie it to the structure. I have noticed bulging of the (flexible) filler I added between the Plexi and the lid. No cracks in the paint yet; too soon to say as I have only done another 50 hours so far.

My suggestion would be to cut and prepare all Plexi in the RV-10 'lid' and make the fit as accurate as possible to leave about a 1/8" (3 to 4 mm) gap around the perimeter. (Between Plexi and the Lid) then bond the Plexi in with Sika and do not paint over the Sika. Now when the Plexi expands or contracts, the Sika will take up the movement. The Sika can be sanded once cured to leave a smooth dull surface. If there is no paint, then it cannot crack. No fg fairing at front lower joint of windscreen, just the sanded Sika fillet.

One builder in the RSA has done this (7 years ago) and I will ask him to send some pictures to post here.
 
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