|
Tissue over Mylar ----
Tissue over Mylar is a method of applying a composite covering to the open framework of models. Its attraction is that the shortcomings of using either mylar or tissue alone are overcome.
First you cover the airframe with mylar, tighten it, and finally use thin dope to attach some tissue on top of the Mylar. Advantages of Tissue with Mylar ---- The big difference using Mylar is that the covering is already airtight before you have even applied any dope, so you really just need enough dope to stick the Tissue or Silk to the Mylar and you can leave it at that. It's quite a bit lighter than brushing several coats of dope, the end result is more stable, it's less likely to warp over time, although warps can be put in easily using a heat gun or hair dryer and twisting the surface. It is also less affected by damp, it won't go slack on wet days. It is a bit more puncture resistant, not a lot, don't expect a bullet proof finish! Like many composite techniques, combining tissue and Mylar makes use of the benefits of both materials, while minimizing the disadvantages. Mylar is waterproof and light, but does not significantly add rigidity to the structure unless you use a thicker and therefore heavier grade. It does exhibit good puncture resistance. Somewhat the reverse is true for tissue as regards mechanical properties. When doped, it adds stiffness to the airframe, but to seal it for air and water, you will add a lot of weight with multiple coats of dope. It is a useful technique for outdoor models in that even with just the one coat of dope, the combination doesn't go slack in the damp. The underlying Mylar also allows easy color changes between the structural elements of the model without doubling upon the tissue. Another advantage is that the mylar seals the inside of the tissue, preventing staining from rubber lube as the model ages. It also provides increased puncture resistance reducing the amount of needed patching as you fly. Both of these factors will help preserve the original appearance of your model. What does it look like? ---- When adding tissue over clear Mylar, it should look like a well done covering job with tissue alone. With tissue on silver the paper looks solid. This is a real advantage for folks who want an opaque finish for a scale model. One difference in appearance is that you can easily make color changes between the structural elements of the model without doubling up on the tissue. Note that on my BA Parasol the red does not lay on top of the white tissue. There is only a small overlap to help keep the edges down. Without the mylar, this type of color scheme would have required adding weight with red paint, having two layers of tissue under the color trim or laboriously joining the sheets of tissue before covering, and praying successfully that they would shrink at identical rates. |
How Long Does it Take? ----
Don't listen to the comments about "covering it twice" taking too long. Much of the time taken to cover with Mylar is offset by the time saved not adding more coats of dope. Also, the tissue goes on the mylar faster than on the bare framework. If the end result is better (and it is) then it's worth a little extra time.
What does it weigh? ---- Here is a surprise, and a good one at that! Properly done, this technique should weigh less than covering with just tissue and dope. How does this work? Dope is really heavy. Several coats are required to fully seal for air and water resistance with just tissue. When adding tissue on top of a light Mylar film, just a single coat of very thin dope will do. The mylar provides all the air and water proofing while the doped tissue adds color and stiffnessMost modelers are using this technique on models to be flown outdoors. Indoor models are often not doped to keep them as light as possible. Adding Mylar will of course add weight. Do I need to add the Tissue? ---- No, but... The tissue adds stiffness and colour. It also makes the model look "correct"! What about warps? ---- Inevitably when covering a model, there are warps that you will want to remove, as well as some you will want to add. Although the film is more plastic than tissue, there is still a slight tendency for it to induce warps when covering. The solution to remove the warp is obvious. Just re-heat the film in the appropriate place, twist and reset the warp. The opposite works for applying warps for trim. Heat, twist and reset. If you have covered the model in aluminised film, because of reflections, it is sometimes very difficult to spot the warp. This is only a problem when straightening the wing before adding the tissue. A bit of persistance will pay off. Cutting the Mylar ---- Cutting and trimming Mylar is best achieved using a sharp new blade, older dull blades may well tear the Mylar. The thinner the Mylar the worse the issue so be careful, but the key is a sharp and clean blade. Also far less tearing occurs you cut tissue or Mylar on one of those green, self healing graphic artist's cutting mats. A rotary knife is also worth a try if you have one. If you haven't got a cutting mat, go out and buy one. They are relatively cheap and will last for years even if you hack into 1/16 ply on them. You'll find your blades stay sharp for longer too. |