Film Covering Techniques By James Goss      If you have read any of my other articles on model building you know that I love to finish my planes with film instead of painting them. I know of no other finishing method that is faster than iron on films such as monokote of ultracote. It takes you from a wood frame to the finished product in one step. It also adds a great deal of tensile strength to your airframe. This keeps your wings from stretching and pulling apart during high stress maneuvers. You may think film does not provide any resistance to crushing forces during a crash, but remember in order for it to crush, something has got to give. So in a round about way film does provide some resistance to crushing.      I always listen with open ears when modelers are talking about covering and building techniques. In the past I have heard several modelers comment that film covering does not add any strength to their models. I guess after you see a model crash and is nothing more than a pile of sticks, it does appear to provide very little strength. What would that model look like after the crash if the balsa was painted and not covered with film? I have used this technique in the past where I would simply stabilize the wood with some type of sealer, add filler, and then paint the surface. The end product looks good but no strength has been added to the airframe. To get strength with a paint job you must first cover the model with some type of covering, such as fabric, film, or fiberglass the model, and then start filling, priming and painting.  Now on a long-term project this would be fine, but if you want to get your model into the air in the smallest amount of time, use film. When correctly applied a film finish will rival that of a paint job and only require half the time and energy.      I will be the first to admit that even though it adds more weight, the ultimate finishing technique is a fiberglass finish. Fiberglass adds both tensile strength and crush resistance to your airframe. It will last for years and years and never have wrinkles for you to deal with. It just takes too much time for me to fiberglass a model. Even with an occasional wrinkle in the film or an edge lifting up here and there, the time saving advantage that film offers is what makes it so attractive to me compared to other methods.      I remember the first model I covered with monokote, I think it was in 1969 and I was using red monokote and my wife's iron to stick it on the balsa. It actually came out looking pretty good to be my first covering job.  So I said to myself, “working with this stuff is really easy”.  Knowing this I started my second project, a glider, and this was a disaster. I had wrinkles on top of wrinkles.  This changed my opinion about monokote being so easy to work with. Since that time, and many covering jobs under my belt, I have become proficient in using film coverings and would like to tell you about some of the methods I use.      Before I discuss two or three of the most important techniques that I use when covering a model, let me say that there are two accepted methods of applying film coverings over wood sheeting. One is to tack the edges of the covering down with your iron and then remove the wrinkles with your heat gun only. Do not use the iron, even with a sock on it.  Not only will using the iron mar and dent the balsa, it will also allow all the wood grain to show through. Using this method will provide a finish that will be as smooth as a paint job. The only disadvantage of this technique is that the covering will have air trapped under it and will not bond to the balsa. You will be going over it quite often with your heat gun to remove wrinkles and tighten up the slack, especially on hot summer days. With time it will finally set and you will see fewer and fewer wrinkles form. Ultracote seems to set faster than monokote.  By set I mean it will stabilize and stop stretching in the heat of the sun. So for a beautiful finish over solid wood use only your heat gun and not the iron.        The second accepted method is just the opposite of the first. The idea here is to let your iron make contact with the covering and press it down onto the surface. If you have a sock over your iron you can use the iron to rub or stroke the covering to force the air, that would be trapped under the film, to stay ahead of the iron and work it to an exit point. You can also use your heat gun to do the same thing, but you will need to go behind the gun and rub the film with a cotton ball until it is cool. In either case this will ensure a good bond between the film and balsa. The wood grain will show through but it will be uniform and not be spotted. Overall it is still a nice finish and will stay put for years to come with only little maintenance.      Remember that the key to a good bond between balsa and film is to vacuum the balsa before covering and make sure you remove all the air from under the film when bonding it to the wood.  A tack-rag is not good enough here. It will not get all the microscopic dust particles like a vacuum will. The film can't bond to wood that has loose dust on it.   An example of this technique can be used when covering an aileron. Cover the bottom first and then the top to hide your seams. Tack the edges but leave one end open. Work toward the open end forcing the air to that end by stroking the aileron with your iron as you go. The results will be a nice finish that is bonded to the balsa and will have minimum wrinkles to appear in the future.      One of the first lessons a new modeler will learn is that film covering will not cover-up any blemishes in the wood. If anything, the film will highlight the blemish and it will show up even more after it is covered. Prepare the surface to be covered by sanding it as if it were going to be painted, vacuum it several times to remove the dust and its ready to cover.      The temperature of your iron determines the overall outcome. An iron that is too hot will create steam and other gasses and they will show up as bubbles. Now every job will have a few bubbles, especially if you are putting film over film, but they will not be noticed after the project is complete. Keeping the temperature of the iron as low as possible to get the job done will give the best results. When working with an edge seam it is mandatory to use low heat that prevents the edge from distorting. I can't count the times I have had a nice cut edge, such as on the turtle deck or forward of the cockpit, on the top center seam and have it distorted by too much heat on the iron.  The only option here is to replace it or leave it as the finished product.      Now I am sure everyone knows the basics of covering with film so I will not bore you with these details at this time. I would think that every covering job at some time or another would have a compound curve, such as a half round wingtip. I consider the rounded edges to be the most challenging of all covering jobs. I do not cover the tips first and then cover over to them like some modelers do. I take the covering around the tip in one sweep. Here is how I do it: Leave plenty of overhang on the film so you can really grip it and pull hard. Don't be skimpy and try to save a few inches here. A common mistake is to not leave enough material after cutting for gripping. Enough heat will allow you to really stretch the film to its maximum. When taking the film around a curve don't worry about steam build up, throw the heat to it. Start out with a moderate heat and see how things go. When it gets to the point where the wrinkles will not work out, increase the heat. Here you must know how much the covering can withstand by practicing on some scrap material first. For both monokote and ultracote you will end up with almost maximum heat on your iron. As you go around the curve start heating the covering before you allow it to touch the wood. It is easier to remove the wrinkles by stretching the film while it is still in the air and then slowly place it onto the wood. A common mistake here is not holding the film tight enough until it cools down sufficiently so it will not let go of the wood when you release it. Keep it stretched tight while at the same time rub it with cotton, or in my case I use my thumb, until its adhesive cools down. Over the years my thumbs have become very heat resistant and I think I could probably place my thumb on the surface or the sun without yelling ----!      You will probably encounter more wrinkles as you get close to the front corner of the wingtip because the radius of the curve is less here. The wrinkles really start to accumulate at this location. Placing your iron on top of these wrinkles may bond then to each other instead of removing them and you will never get them apart without damage to the covering. This will really look bad and you will probably want to tear the covering off and start over. While gripping and stretching the film as much as you can, apply heat from your gun or iron. When you get to this point of what I call “high density wrinkles”, do not use your heating iron, instead use your heat gun and watch the wrinkles disappear. Remember that at this time you are still holding the film above the wood, so now slowly bring it down into contact and hold in place for about a minute before you let it go.        Once the covering is around the curve the next important step is to cut the excess off. Now you should have plenty of excess to cut off at this time because you wanted to have plenty of material to grip and pull. If you only leave one inch over hang you will not be able to work the material the way it should be. I like to leave about four or five inches for gripping. When cutting the film the main thing to remember is make sure you cut in a straight line. Even if this is the bottom of the wing being covered first and will be covered over later, a jigsaw cut will show through the top layer and distract from its appearance more so than a straight cut will. Getting this straight cut around compound curves is not easy. With ultracote I like to get the material tacked down beyond where I really need it and place masking tape to form the outline of the cut I am looking for. With a new knife blade in your hobby knife you will be able to cut through the top layer without cutting into the bottom layer of film. Just follow the tape outline. You may need to practice a few times on scrap but you can do it. By bonding the film first and then cutting you will have a very neat cut line. If you can arrange to have the cut line on an edge line of the frame the cut seam will absolutely not be visible. This is the real secret for a professional finish. I have had modelers look for seams on some of my better jobs and couldn't find them.      One of my most valued tools that I use for covering with film is what I call a heat sink.  This is nothing more than a piece of sheet metal, any gauge, that I use to keep heat away from any unwanted area of the film. I have several sizes to select from when I need them. A piece of metal about 5 x 10 inches will work on most jobs.  As an example of using your heat sink, lets say you are going to cover the front half of the wing in one color and the back half in another. You will cover the back half first and anchor it to the sheeting on the leading edge just above the main spar. Do not try to splice the two films over an open rib bay, make sure it is over the sheeting. Tack the leading edge film to the rear film using only low heat with about a 1-inch overlap and proceed to secure the rest of the edges with your heat iron. Don't forget to leave an exit for the air to escape. Using your heat gun start at the root of the wing and work toward the tip.  Anytime you get close to the splice lay the heat sink over the edge of the splice and you can work that edge without any distortion at all. This technique really does work.  Metal being a good conductor of heat works a lot better than trying to use a nonmetal. By sinking the heat away from the splice it ensures that the film's edge will not pull loose.  Try this on your next covering job and you will love the results.      Remember when working with film to use the lowest heat setting possible and still get a good bond to the wood. Once the film is glued to the wood you can go back over it if necessary with a little more heat. Too much heat and not having an exit for the air to escape are the main reasons that we have bubbles in the finish. Here is a good tip if you are covering an open tail section or an aileron that is not sheeted. Create an exit point for the air in these structures by drilling a small hole in the trailing edge of each surface. If you don't, you will see the air expand the covering when heat is applied, I call this ballooning. When the air cools back down there will be slack in the covering. Anyone that has covered a few tail sections will know what I am describing here. The tiny holes will be covered last when you bring the top covering over the trailing edge and will never be seen. I use a tiny drill bit in a handheld pin-vice because you may need to drill through some of the ribs and with the pin-vice you can get into close quarters. This really improves the overall covering job.      The last tip for covering with film is when working with the tail section where you are going to but the horizontal stabilizer's film into the fuse or vertical fin. Always start by making a fillet to go where the two pieces of material will join. A fillet is nothing more than a small strip of film cut about 3/8 of an inch wide to fit into the corner of where the two materials meet. Without this strip of material in place, the two coverings may shrink away from the joint and balsa may show. I like to do all the fillets before I start covering the large surfaces. After the job is complete you will not notice these fillets, they will blend in and look like one solid film.      I am sure everyone that has been covering with film for some time will certainly have his or her own technique for covering. Like I said, over the years I have learned a lot just by listening to other modelers. If you have some good tips why not do an article because everybody knows something that is worth while for others to learn. Good luck on your film covering techniques.

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