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 10:08:24 PM UTC+2, Roy B. wrote:
1) Are you saying you actually sand the Mylar taped surface itself to improve adhesion?

Please do not sand your wings, the mylar will adhere quite well to normal surface.

I've done the steps that Bob K mentions below a little differently....

Once the tape is in place and the mylar is perfectly positioned on top of the tape (protective plastic still in place on the tape) I don't flip up the mylar. I leave a 3-6" long strip of the protective film sticking out from under the right or left end of the mylar and I pull the protective film out from under the mylar while carefully holding the mylar in position. Easy enough to practice this on a short segment first, but it avoids anything moving or going "off" when you try to flip a long section of mylar back down onto the, now exposed, adhesive. Just carefully pull a few inches of the protective film out from under the mylar at a time while holding everything stable.


* 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.

--Bob K.

On Thursday, March 11, 2021 at 6:29:36 AM UTC-8, wrote:
I've done the steps that Bob K mentions below a little differently....

Once the tape is in place and the mylar is perfectly positioned on top of the tape (protective plastic still in place on the tape) I don't flip up the mylar. I leave a 3-6" long strip of the protective film sticking out from under the right or left end of the mylar and I pull the protective film out from under the mylar while carefully holding the mylar in position. Easy enough to practice this on a short segment first, but it avoids anything moving or going "off" when you try to flip a long section of mylar back down onto the, now exposed, adhesive. Just carefully pull a few inches of the protective film out from under the mylar at a time while holding everything stable.
* 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.
--Bob K.

To sand or not to sand..........that is the question?
Rather than restating my Mylar procedure, let me tell you how and why I developed it?
I installed wide Mylar on a ASW-20 and 2 weeks later the ship was sitting back in my shop. Owner said he thought he’d take her up to redline...............ship yawed quite a bit to the right, then straightened out as all Mylar on the right wing disappeared! Owner also showed me the lower Mylar on the left wing was hanging by the safety tape! We decided the left wing Mylar had pealed off caused by flaps not locked in neutral then left in a hot trailer!

No reports of shedding Mylar since we started sanding Both surfaces with 220 grit............some 40 years ago!

You decide?
JJ