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Author Topic: Covering With Mylar  (Read 2687 times)
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Yak 52
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« on: March 23, 2012, 11:09:57 AM »

This discussion arose out of a build thread here: http://www.hippocketaeronautics.com/hpa_forum/index.php/topic,11662.msg78692.html#msg78692

But it would be good if we could discuss techniques for covering with Mylar in more detail. I've never used it but I'm interested in having a go - one because I have a few models in mind that are polished aluminium and silver mylar would be just the job, and two I'm building a 32" span RC trainer which could probably do with something a bit strong than tissue (I've never used any of the plastic films either)

Jon
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DerekMc
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« Reply #1 on: March 23, 2012, 11:30:12 AM »

Several years ago I put together a tutorial on covering with Mylar and tissue over Mylar.  You can download it from the Hip Pocket Plans page.

http://www.hippocketaeronautics.com/hpa_plans/details.php?image_id=240

It has lots of pictures Grin

An important consideration is that not all mylar films are equal. The mylar you have might take more or less heat to shrink or dye. There is a technique learning adaptation process!
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« Reply #2 on: March 24, 2012, 01:16:24 PM »

Mylar and tissue over mylar has been used for some years.  I was an early convert after visiting Mike Woodhouse in Norwich at least 20 years ago I guess.

I haven't looked at the SFA article as it is only open to registered users so can't comment on the techniques used in that article.

My mylar comes from either Mike Woodhouse or John O'Donnell.  Flitehook may also sell it.  These sources are UK based. I and others always used to use Evostic contact adhesive thinned with toluene or cellulose thinners.  I tried the latter but have stuck (pun intended) with toluene.  When Evostic changed their formula for eco reasons it became unusable.  I have found an excellent alternative in the UK called Maxigrip and bought toluene off ebay sellers.

A very sharp modelling knife or double edged razor blades, that can be broken using small pliers to give a sharp point, are needed to cleanly cut mylar without it rucking and tearing.

Prepare the airframe to be covered by doing all the necessary sanding to a slightly higher standard than I found necessary for straight tissue covering as the mylar shows up all the little uneven joins and oversanded areas.  Then I apply very thin dope/thinners to the areas that are to be in contact with the mylar. This raises the grain and any loose fibres and when the dope is thoroughly dry smooth over with the finest abrasive paper, preferably well worn. This should give a slightly shiny and sealed surface.  Get rid of the dust using a soft brush, blowing, vacuum cleaner (carefully) and put the parts aside.  Use a very sharp pin to pierce holes in ribs and closed cells to allow trapped air to eventually vent to atmosphere.

The bench or table has to be big enough to allow the mylar to be unrolled and be dust free. I cover the bench with old clean newspaper, unroll the mylar and either draw around the airframe parts or use paper templates to enable the seperate pieces of mylar to be cut out. For this operation a very sharp modelling knife or razor blade is essential. Leave about 1/2" margin around the edges. It is important to get the pieces cut out with the "grain" in the right direction. If using silver mylar decide on shiny side or dull side to be visible. 

Wings and tailplanes need the covering to be cut so that the spanwise direction is along the length of the roll. This reduces the sagging between the ribs. I think that the mylar is more stretched along it's length than it's width during production and shrinks more along the length when heated. I may be wrong about the reason but am sure about the effect.

I cut and cover in stages rather than cut everything, pile it up and sort out the correct pieces from a pile.

I use an old cut down 35mm film container to prepare the adhesive when I am ready to cover  the airframe.  Only make up enough thinned adhesive to do a wing, a tailplane, etc., as the solvents are volatile and smell. The warnings on the adhesive and solvent containers show that they can have a nasty effect on the user.  The indications are that no-one should use this stuff.  So it is up to you!

Some sort of small electric iron is needed to reactivate the adhesive, firm it down and finally shrink it.  Before you tackle a model it is worth doing some trials at this stage to find out what maximum indicated temperature your iron can be set when heat shrinking.  As a rule of thumb I find that silver mylar will only take half the heat setting compared to clear mylar on my iron.  Any more and it self destructs.  For tacking the edges I use about half the max setting for the type of mylar being used.

Choose a calm day to cover and leave the workshop door open. If your wife/partner complains about the smell of dope they will complain about this adhesive.

To mix the adhesive I put a small drop, about the size of a pea, of adhesive from the tube into the cut down film container and use an "eye-dropper" to introduce the solvent. Use a wooden cocktail stick or similar to stir and keep introducing solvent until it is as thin as water and drips off the cocktail stick. It needs to be this thin to easily brush on.  It is ok to introduce more solvent to keep it thin.  Have a small bottle with small neck with some solvent to dunk the brush and wipe with a paper towel so that the adhesive doesn't harden the brush.

You now have, let us say, a wing or tailplane with top and bottom mylar cut pieces.  I cover the bottom first so carefully brush the airframe with thinned adhesive using a small paintbrush about 1/4" long.  Try to keep the adhesive away from the inside edges of the LE and TE so the mylar doesn't stick where it shouldn't and introduce wrinkles. Allow the adhesive to dry (5 - 7 minutes depending on temperature) the carefully load the mylar trying to get it to lay evenly without sags and wrinkles.  You won't do this without pulling off and re-applying, hence the need to allow the adhesive to be non-tacky.  Use the iron at the low setting to tack the mylar along the LE and TE.  Lift and re-apply as necessary to keep it even. Finish tacking including ribs and spars, etc.  When it looks okay, turn up the heat and after the iron has stabilised at the higher safe setting, shrink the mylar keeping the iron moving along the length and across the chord of the component.  Don't dwell or you may have to start again. Don't try for maximum shrinkage at this stage. Smooth the mylar over the LE and TE.  Use a very sharp razor blade to trim the mylar and admire your efforts. Use the iron to smooth down any little snags. Reduce the iron temperature to tack setting.

Apply adhesive to the top surface and repeat the above.  Make sure the adhesive covers LE and TE to overlap the bottom surface mylar.  When shrinking the mylar pay particular attention to the area on wings just behind the LE.  If this area is not properly shrunk it will wrinkle later.  Once again, do not go for the final shrink at this stage.  Pierce into one of the interconnected closed cells to provide vent to atmosphere.Weight or pin the wing down to the bench introducing warps as necessary and leave overnight.  Next day give the covering a final shrink and pin down again.  With average luck the wing/tailplane will stay that way.

Fuselages can be done as above.  If it is a scale model there is likely to be curved surfaces and possibly double curvatures.  See the Seamew and Storch pictures. These both have double curvatures around the fin to fuselage junctures.  For both models the roll length direction is vertical on the model.  If the mylar had been cut to have the roll length along the fuselage length the mylar would have shrunk to have a hollow shape.

Now decide if you want to leave it as clear mylar or clear mylar with a turbulating dusting of spray can acrylic or leave it as shiny or dull silver mylar with or without a sprayed finish. Another alternative is to cover clear mylar with tissue and either leave in it's natural color or paint. Choices, choices.

Let's assume that you know how to apply tissue to mylar or can find out.  There are different ways that I have read but I apply Esaki tissue shiny side down, applied wet.  If you need more information please ask.  To give a dusting of Spray acrylic to clear mylar - heat the can in hot water, lightly wipe over the surface to be painted with toluene or other solvent and gently spray from about12" to give a roughish finish.

To get an Airfix model type finish to dull-side out silver mylar covering I wipe over the surfaces with solvent.  I use good quality model type acrylic paints, thinned with tap water.  Not too thin, but you need to get it just right. I use a Badger spray gun run from a small compressor.  Usually give one thin coat that hardly covers the surface then when that is nearly dry another similar coat to cover.  I use matt paints and finish with a very much thinned clear gloss varnish to take off that very flat dull effect.  Even drab military aircraft have a bit of a shine on the tops of fuselages, etc., when viewed from a distance.  I have read that a drop of white added to the paint gives the model an appearance of being viewed from a scale distance.  Too glossy or too drab models don't look right.  Consult an artist or read some books on model finishing. 

The pictures show
--- a very well flown Auster AOP with yellow tissue over clear mylar and a very light dusting of yellow acrylic. This has a     
     see-through   appearance that is non scale like to me.
--- a  fairly well flown Feiseler Storch fuselage and wing.  White tissue over clear mylar painted with sufficient acrylic so that it is
     opaque.
--- a not flown Short Seamew (one of the ugliest, slowest and noisiest aircraft ever) with silver mylar dull side out painted with
     acrylic and was done to check the technique.
--- a not flown Westland Lysander covered in unpainted silver mylar dull side out to represent the first prototype.  I think that to
     look right it needs a light coat of silver acrylic to hide the stretch marks.
--- a not flown Guillows Bird Dog covered in silver mylar dull side out painted with Testors acrylic paint. 

Although some of the above are shown as not flown (mainly because of the awful weather in the UK last year) I have a fleet of P30, Coupe, Vintage models, etc.,covered with most of the techniques outlined that have done a lot of flying.  I think that the Seamew and Bird Dog type finishes could suffer a bit from expansion in places with a lot of sun but are okay in Northern Europe and for indoor flying.

Apologies for the length of above.  Best of luck.


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Prosper
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« Reply #3 on: March 29, 2012, 11:56:23 AM »

Thanks for the explanation Spadge. I think it might be winter before I start experimenting with this stuff. I've been mulling over a design for a 22" DHC-2 Beaver on and off for a year or so. Ideal candidate I reckon, especially as I'd chosen the silver and red U.S. Air Force scheme.
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« Reply #4 on: March 29, 2012, 02:56:20 PM »

Thanks for the explanation Spadge. I think it might be winter before I start experimenting with this stuff. I've been mulling over a design for a 22" DHC-2 Beaver on and off for a year or so. Ideal candidate I reckon, especially as I'd chosen the silver and red U.S. Air Force scheme.

Go for it. It is easier than you think.  I see that I missed a "silver" model with the photos.  It is a Keil Kraft Lysander but with a bit of a cheat in that I used the "Lost Foam" method of making the fuselage.  I think that it came from SFA but I lost all those threads and can't remember the name of the USA based gentleman who kindly sent me details of how to do it.
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Yak 52
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« Reply #5 on: January 13, 2013, 08:23:52 AM »

Hi chaps,

I'm finally hoping to experiment with mylar, but I'm not sure what thickness to order. The model will be a 36" glider with tissue over the top. And I'm a total rookie... any suggestions?

Thanks,
Jon
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« Reply #6 on: January 13, 2013, 09:15:27 AM »

Jon

5 micron should be more than sufficient with some lightweight tissue on top. Most recently I've used that combination on a porky E36 (170+ grams) and will do the same with the 36" vintage bungee launched glider I'm in the throes of building.

Happy shrinking.

Peter
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« Reply #7 on: January 13, 2013, 11:54:46 AM »

Hi Jon

I agree with Peetee that 5 micron is thick enough, but the thicker ones are easier to handle without too much of a weight penalty.

(On all my gliders (4' upwards) I now use 10 micron under esaki)  Roll Eyes

The last couple of gliders I've built (circle tow F1H and Nostalgia glider) both had 10 micron under medium/heavy weight esaki. For rubber jobs of around the size you are talking about I use 5 micron and lightweight esaki.

Toodlepip
paul

ps - Some nice models there Spadge and an excellent explanation of how to cover  Smiley
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« Reply #8 on: January 13, 2013, 04:36:54 PM »

Ok thanks, I'll have a try with 5 micron.

Any glue recommendations? Does anyone use the U-glue Mike Woodhouse sells?

The iron that Spadge mentions - I presume that's a film covering modelling type? I don't have one of those, so are there other methods?
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« Reply #9 on: January 14, 2013, 03:48:25 AM »

I've found thinned contact adhesive to be far superior to the likes of Balsaloc (although mine was a few years old). As I said elsewhere, solvent based contact adhesive from Wickes or the pound store works well, diluted with cellulose thinners. The solvent based stuff normally has a "Highly inflammable" warning label, and my current lot is branded 'Edwards' from Pound World (and I did some mylaring with it last night.

You can use a domestic iron. I used to but switched to a cheap r/c film iron to avoid domestic conflict Roll Eyes I can't remember the heat settings I used so it's best to experiment on a sample structure.

Peter
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« Reply #10 on: January 18, 2013, 02:51:52 PM »

Thanks for comments - glad to report that the Bird Dog flies quite nicely.

I use a small "Travel iron" for shrinking all heat-shrink coverings.  I have one of those small specialised heat-shrink covering irons but never use it as the iron sole plate is on the end of a stick and more difficult to position precisely. A larger sole plate makes it easier to cover larger areas.  There are travel irons on fleabay for very little money and I would recommend one of those. 

I agree with PeeTee that contact adhesive is preferable to Balsaloc (looks and smells like PVA and probably is. Somewhere in the archives here are comments on re-activating PVA with heat - it follows).  The Maxigrip adhesive that I referred to in my description is no longer available.  I phoned the supplier who told me that H&S have banned some of the ingredients.  So following PeeTees advice I will visit Pound World and/or Poundland for a couple of tubes of Edwards before that too is no longer available. The Maxigrip man also said that one of the ingredients of all Cyano type adhesives is in the process of being banned so I will stock up on a couple of bottles of that at the same time.
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« Reply #11 on: May 16, 2013, 03:48:21 PM »

Thixofix also works well if you can find it, for thinning I use Evostik cleaner. I had a long correspondance with Mike woodhouse when Evostik got Elfinned,( elfin safety) Nigel assures me we can go back to the full strength stuff when  he's in charge. I have to say I find the new stuff works fine thinned down. Just make sure the glue is still wet when you apply the Mylar and shrink it with a hair drier. You can lift locally and adjust several times to get big wrinkles out. If you use contact adhesive as a contact adhesive and let it dry out before applying the film in my experience this is when the problems start with the new stuff. When you put the tissue on make sure every where is overlapped so the film is fully enclosed. I lay the paper on the film and dope through the tissue direct to the film, any paper that wets out well is OK, avoid anything that resembles M.O.D issue IZAL TP. in life and in modelling.It is a good idea to put adhesive on every surface that contacts with the covering. If you suffer puncture damage you can cut the panel out and add a new one. As the mylar stops the dope from reaching the structure the tissue only technique of only attaching the covering to the outer profile and allowing the dope to soak through and attach the rest will not work and the adjoining panels will slacken and bow if they are not attached to the structure when you cut away damaged panels. You can get round this with a bit of fiddling around but why not plan ahead and make life easy? The attached was covered as discussed here.
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« Reply #12 on: May 16, 2013, 04:08:02 PM »

Capnbob that photo brings back memories! I had the kit for the Velox many years ago and I remember sanding all of the balsa blocks and fitting the canopy etc. What I cannot remember is what happened to the model and if I ever flew it! But before I saw your photo I had forgotten about that model completely......


Tmat
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« Reply #13 on: May 16, 2013, 06:11:16 PM »

HI TMat   Don't be tempted by nostalgia, every example of the Velox I have ever seen proved to be a disaster.Why I was tempted heaven only knows. I saw Phil Smith, the designer, carry away a bag full of bits  years ago and when I showed up at a  SAMS meet with mine several people said how unpredictable theirs had been. Mine proved to be no exception. It was an interesting build starting from scratch without all the spindle moulded parts. You are right the fuselage is carved out of tree trunks but the completed model is a pretty thing.Pity about the flight charecteristics!

Notalgia's not what it used to be.
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