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A few points:- 1. Any joints you are covering, overlap a reasonable amount, at least 1/8". 2. Undercamber is not a problem. Just make sure all the structure has adhesive on it, and carefully attach the film to the ribs. 3. The heavier grades of film shrink less than the lighter grades. 4. Make sure that all edges and joints are fully sealed. If opened to the air flow, the air can get in and cause the film to lift away from the structure. 5. Clean the finished covering and remove any adhesive that is on the surface with a small amount of solvent. Not only will the covering look better it will also prevent dirt attaching itself to the model when it is out in the fields. Before heat shrinking the film, a quick test will find if your iron turned up to maximum heat will melt the Mylar. Howver, be aware that if attaching aluminised film to carbon, heat can build up in the structure and spoil the aluminised finish. Also keep the shoe of the iron clean free of adhesive and dust, a quick wipe with solvent will do the job. Adhesive on the shoe will attract dirt which may scratch or even tear the film. Recovery ----Sometimes, even after much effort, wrinkles still remain. If this is the case try leaving the offending item for 24 hours then try again. It seems that if it is left Mylar will recover its shrinking powers and a wrinkled covering job can be ironed out. Securing the Tissue ---- When applying the tisue or silk, it should be damp so as to conform more easily to the undulations and curves of the mylar surface. Once it is positioned as desired, flow on a thin coat of Nitrate dope cut to 50:50 or more. Once this coat dries, it is likely that the dope will have "blushed" a bit. The offending whiteness is a result of the moisture in the covering, and is easily removed by wiping on another light coat of dope, or even just making a pass with thinners. For rubber models and gliders a second coat after a bit of sanding is all you need. Glow and diesel models might need another coat or two to protect the tissue from the fuel and exhaust. Since you are using a highly thinned dope, the weight gain will be minimal. Working With Silk ---- Silk on Mylar also works well, and is very popular with some folks looking for a lighter covering for their models when silk is required. As mentioned above, it should also be applied while damp. |
Sources for Mylar ----
Suitable Mylar is available from many sources. See the addresses below.
Mylar is the trade name given by DuPont of the U.S.A. to a polyester film. Mylar is also sold under other trade names such as Melinex. The film is unaffected by moisture and most other common chemicals. It is also temperature stable unless heated to around 200°C+ when it will shrink. The film can be attached as a covering and then shrunk to give a smooth and tight finish. The film is available in both clear and aluminised finishes. The aluminisation of the film makes it gas tight due to the size of the molecules of aluminium even helium cannot get through. Remember that the aluminisation process of the Mylar reduces to some extent its ability to shrink under heat.
The best Mylar to use on all sizes of models is the thinnest, 5 micron. It is really only there to avoid the use of excessive amounts of dope to seal the tissue, and to give a degree of stability to the covering, using thicker Mylar adds some rigidity, but I think this is better achieved by using a heavier grade of tissue, or using Silk. Thicker Mylar is harder to work around curves, and appears to delaminate easier being less flexible.
Here are the names, email addresses and web sites for the modelers who shared the information that became this guide. Note that Michael J Woodhouse sells FF supplies, including Mylar. Name Web Site Michael J Woodhouse Mike@freeflightsupplies.co.uk www.freeflightsupplies.co.uk Graham Knight graham@studioseven.freeserve.co.uk website.lineone.net/~raynes.pk.mac Martin Gregorie Gregorie@logica.com www.gregorie.demon.co.uk/freeflight/ Tapio Linkosalo linkosal@cc.helsinki.fi Maris Dislers maris.dislers@sawater.sa.gov.au Thayer Syme thayer5@mindspring.com http://www.gryffinaero.com/models/ffpages/tips/mylartissue.html/ |