Mylar Project

I am wondering if some of the more experienced repair guys on this list would share their wisdom on replacing mylar seals. There is lots of stuff on the internet about refinishing a sailplane - but very little about gap seal replacement which at first glance seems easy to do but actually is difficult to do well and professionally.

What are the best products? Sources? Costs?
Best way to remove old seals?
Surface preparation?
Professional application techniques to get a good result?

If this all has been written on before, please direct me to it.
Thanks
ROY

On 3/10/2021 1:49 PM, Roy B. wrote:
I am wondering if some of the more experienced repair guys on this list would share their wisdom on replacing mylar seals. There is lots of stuff on the internet about refinishing a sailplane - but very little about gap seal replacement which at first glance seems easy to do but actually is difficult to do well and professionally.

What are the best products? Sources? Costs?
Best way to remove old seals?
Surface preparation?
Professional application techniques to get a good result?

If this all has been written on before, please direct me to it.
Thanks
ROY

I've looked into this recently on old RAS postings and found three
relevant threads:

https://groups.google.com/g/rec.avia...c/c_MZ2k2kRY4/

https://groups.google.com/g/rec.avia...c/414xDarWxTc/

https://groups.google.com/g/rec.avia...c/1TAbxX3TOJE/

Of course, the comments therein contradict each other.

Not an experienced repair guy, but I have put seals on a couple different gliders. This is how I do it:

* Sand the adhesion areas on both the wing and the underside of the mylar 220 grit.
* Degrease both surfaces with acetone. Repeat as necessary if you happen to touch or otherwise contaminate any of the adhesion surfaces.
* Apply the adhesion tape first to the wing, leaving the protective strip in place on its back side.
* Lay the mylar in place and shift it as necessary so that it is perfectly straight along the wing above the adhesion tape. It is very stiff, so the slightest deviation results in a kink that resists conforming to the curve of the airfoil.
* When the mylar is located properly, temporarily tag it in place with bits of masking tape or glider tape every foot or so.
* With the mylar tagged in place, flip the mylar forward over the tags, using the tags as hinges. Then tag the mylar down in this position with a few more tags of tape.
* Peel the protective strip from the adhesion tape.
* Carefully flip the mylar back over onto the adhesion tape, taking care that it arrives in the position where it was originally tagged down.
* Use a small ball bearing, or better yet a pair of ball bearings (I use 626 bearings) bolted to a handle to aggressively roll the mylar down onto the adhesion tape, and the adhesion tape down onto the wing. You should see the color change through the mylar as you get full contact between the adhesion tape and the mylar.
* Remove the temporary tape tags.
* Apply the thin white PVC safety tape along the leading edge of the mylar so that it is half on the mylar and half on the wing.

JJ has done this much more than I, he probably has some additional advice.

--Bob K.

One other tip: Don't try to install the mylar seals if it is cold in the shop! The tape won't adhere very well at all and you will have to do it over when the seals blow off on your first flight.

Bob:
Thanks!

2 Questions/Clarifications:

1) Are you saying you actually sand the Mylar taped surface itself to improve adhesion?

2) I have something called a "wallpaper seam roller" ( a 1" wide hard roller on a handle) that seems it will do the job of you 2 ball bearings. Does that make sense?

ROY

On Wednesday, March 10, 2021 at 3:08:24 PM UTC-5, Roy B. wrote:
Bob:
Thanks!

2 Questions/Clarifications:

1) Are you saying you actually sand the Mylar taped surface itself to improve adhesion?

2) I have something called a "wallpaper seam roller" ( a 1" wide hard roller on a handle) that seems it will do the job of you 2 ball bearings. Does that make sense?

ROY
Good description from Bob. I have had better results when I scuff the mylar with 220 as Bob describes. I'm not sure whether the roughness or the cleaning is the thing that gives the benefit. I NEVER use paper towels when doing the final wipe due to solvent possibly leeching contaminants from the paper. Just use a clean cloth shop rag. In use a wood block with a 1/8 radius sanded in the end to rub down. Use what works for your hands to apply firm even pressure. Make sure controls stay neutral until the adhesive cures.
One other thing from experience. You may be tempted to buy seals with adhesive per applied. In every case I have encountered them they were curved and would not lay down straight.
Have fun
UH

On Wednesday, March 10, 2021 at 12:08:24 PM UTC-8, Roy B. wrote:

1) Are you saying you actually sand the Mylar taped surface itself to improve adhesion?

Yes, I sand the underside of the mylar where the tape goes. It seems to have no ill effect on looks or functionality, and it definitely improves adhesion. As JJ has noted in other posts, you really only have to sand the portion where the adhesive tape goes, but it does not hurt if you sand more area than that.

2) I have something called a "wallpaper seam roller" ( a 1" wide hard roller on a handle) that seems it will do the job of you 2 ball bearings. Does that make sense?

That tool will probably work just fine. However, given how much pressure it takes to do right, and the sheer length of the mylar seal, you might wear the tool out before you're done.

--Bob K.

On Wednesday, March 10, 2021 at 4:41:53 PM UTC-5, Bob Kuykendall wrote:
On Wednesday, March 10, 2021 at 12:08:24 PM UTC-8, Roy B. wrote:

1) Are you saying you actually sand the Mylar taped surface itself to improve adhesion?
Yes, I sand the underside of the mylar where the tape goes. It seems to have no ill effect on looks or functionality, and it definitely improves adhesion. As JJ has noted in other posts, you really only have to sand the portion where the adhesive tape goes, but it does not hurt if you sand more area than that.
2) I have something called a "wallpaper seam roller" ( a 1" wide hard roller on a handle) that seems it will do the job of you 2 ball bearings. Does that make sense?
That tool will probably work just fine. However, given how much pressure it takes to do right, and the sheer length of the mylar seal, you might wear the tool out before you're done.

--Bob K.
The wallpaper seam roller works fine; I use the same proceedure described above for aligning
the mylar.
The section behind the dive brakes never stay on for a season; as a test i tried Shoogoo as adhesive
it makes a better bond but is more trouble to apply. It needs 24 hrs and 20C to cure as the
solvent has to evaporate from the seam.

JMF

What others have described mirrors my technique but I lay masking tape the full length of the mylar span-wise and crease it sharply at the leading mylar edge using the "handle" of a single-edge razor blade to form the hinge. I do not sand either surface but surgically clean both, first with acetone and then alcohol and clean, cotton cloth. I then never touch the bonding surfaces. The red Tesa transfer tape is best, in my experience. Fresh transfer tape and mylar is preferred. Pre-applied mylar/transfer tape has not worked for me.

Removal is a PITA. The transfer tape may come off with the mylar if it is not ancient and your heart is pure. Be careful not to peel up paint. If not, I then try rolling it up with fingers. If these fail, then solvents and potentially a gooey mess is next. 3M general adhesive remover is best, imo. Get the liquid in a can and place it in a squeeze condiment bottle. I cut paper towels into strips and lay it on the offending tape. Soak with solvent and wait. If is a warm day, I cover it with plastic qrap to slow evaporation. I shear up the softened transfer tape with a chisel-shaped tongue depressor.

It is possible that the trailing edge of the wing is not span-wise straight.. It is also possible that the mylar will not lie straight span-wise. There is essentially zero "give" in the mylar. Let it lie where it wants and masking tape the leading edge down. An extra pair of hands is valuable. Don't touch the bonding surface or let them touch anything else post-cleaning.

Post-Application: The ship and materials need to be warm (70-80 degrees) and kept warm and unloaded post-application for a day or two (more is better). I keep mylar seals unloaded in the trailer by using control locks. I have seen 3 brand-new gliders experience 100% top surface mylar debond since the controls lay in the trailers during shipment at full deflection, keeping the them loaded.

Removal is a PITA. The transfer tape may come off with the mylar if it is not ancient and your heart is pure. Be careful not to peel up paint. If not, I then try rolling it up with fingers. If these fail, then solvents and potentially a gooey mess is next. 3M general adhesive remover is best, imo. Get the liquid in a can and place it in a squeeze condiment bottle. I cut paper towels into strips and lay it on the offending tape. Soak with solvent and wait. If is a warm day, I cover it with plastic qrap to slow evaporation. I shear up the softened transfer tape with a chisel-shaped tongue depressor.

I also use solvent soaked paper towel strips to saturate the old adhesive tape. Surprisingly, Goo-Gone works pretty well, and smells nicer than 3M adhesive remover. It also isn't quite as nasty as the potent "Methyl Ethyl Deathyl" removers. Use nitrile gloves to keep from contaminating the surface with skin oils. Gloves also prevent skin absorption of the aforementioned solvent and the sticky, gummy, snot-like adhesive doesn't stick to your fingers. And for scraping the residue, I use plastic single-edge razor blades.

(https://www.amazon.com/Single-Edge-P...s%2C285&sr=8-7

After scraping and a wipedown with acetone, look carefully at the surface in reflected light to find any spots you might have missed. Then scuff with 200 grit (both the surface and the mylar) and wipe down again. Let the acetone evaporate completely (20 minutes or so).Try to keep any dust from settling on the surface- meaning avoid doing this in a drafty shop with airborne particulates drifting around. Apply fresh Tesa double sided tape (with the pinkish colored plastic backing). I use the edge of the control surface cutout at the rear of the wing as a reference when sticking the tape in place.. Use a roller or a ball bearing as described previously to firmly press the tape against the wing surface until it is uniformly settled. As you press with the roller or ball bearing, you can see where the tape sticks by the color change- it appears a bit darker visually.

The next step seems to have worked for me pretty well. Stretch the new mylar along the forward edge of the Tesa tape (with the pink backing still in place) and tack it into place with masking tape or the semi-sticky blue painters tape at about 12-18 inch intervals. You should be able to avoid any curvature in the line if you are careful. Then, you can lift the end of the mylar and peel the Tesa tape backing off to the side at a 90 degree angle. Pull the backing off to the side while running your fingers along the mylar just behind the backing as it peels away. This keeps the mylar positioned exactly where it should be and prevents it from curving off track. Peel off each strip of masking tape that is holding the mylar in position as you come to it and, when it is finally stuck down, use the roller or ball bearing to press it firmly. Extra effort in pressing it down now will be worth it. Make several passes to be sure you got it right. If it is a bit chilly, a heat gun can be used to raise the temperature of the adhesive until it is warm, (but not hot) as you press the mylar down.

And then leave. You want the adhesive to sit undisturbed for at least a full day to let it set up. Finally, use the very thin Tesa PVC safety tape overlapping the leading edge and the wing surface to keep the leading edge of the mylar from peeling up in the airflow.

I've had pretty good luck doing it this way. Every time I've tried to rush it or take shortcuts, I end up having to do it again. I have also found that you simply can't re-use mylar. Once you peel it up, it will never lay flat and straight again. Do the right thing and order new stuff. You'll save by not having to spend the extra time peeling it up and going through the whole adhesive removal/cleaning/scuffing/reapplication process again.