Covering Indoor Models with Plastic Films,
Using the Thinner Covering Films
An article from Indoor News and Views (INAV)
This article is from Indoor News and Views (INAV) newsletter and had no author attributed to it. Posting on this website is with the permission of the editor of INAV
This article will cover using polymicro II or the new Y2K films to cover indoor models. The newer films are more delicate than previous films and so demand new techniques to get good results.
The first step in using the plastic films for covering is to make a covering frame. Basically a covering frame is a pair of rails that the covering is glued to that hold the film while a structure is lowered onto the covering. A method of adjusting the rails in relation to one another is needed to produce the slack that allows the structure to be covered without the film being too tight. Any simple system to allow one rail to be adjusted will work. One method is to use 1/4" x 1 1/2" X 24" hard balsa rails that have two slots cut into each of them 2" from each end and in the middle of the 1 1/2" face. The slots are cut to be a tight fit for 1/4"X 1/2"X 7" spreader pieces that tie the two rails together. Once the rails have the spreader pieces inserted into them the position of the rails can be adjusted by simply pushing the spreaders into or out of the slots until the desired film slack is achieved and any diagonal wrinkles are removed. A better but more elaborate system is to build a pair of rectangular frames from 1/8" X 1/2" X 35" as shown in the diagram. These are joined by 1/8" X 3" X 7" hard balsa crosspieces along the bottom only. This leaves the upper rails of the frames very flexible. A set of five turnbuckles are used to adjust the width and diagonal tension on the upper rails. These upper rails are the ones used to hold the film. Three of the turn buckles go across the frames, one in the center and one at each end. The other two turnbuckles are set up along the diagonals of the frame and are used to adjust the diagonal wrinkles out of the film. This frame is more permanent and more versatile than the frame first described, although both do a good job.Getting the film off of the card tube is not a trivial job, especially the new Y2K film. Gene Joshu discovered very recently that if you unroll the film out onto a large piece of the foam rubber used in furniture upholstery a very large part of the static that plagues this material is eliminated. Once the film is unrolled to the desired length a piece of cardboard is placed under the film at the desired cut off point. A hot soldering iron of about 15 watts is recommended for film cutting and trimming. Once the film has been cut you can further eliminate static by gently wrinkling the film by rolling it up into very loose ball about 3" in diameter and massaging it gently with the fingertips for a minute or so. Do not roll the ball too tight, this can ruin the film by bursting small air bubbles. Gently and slowly unravel the film onto a smooth matte surface such as a piece of cardboard or Formica. If you are using the new Y2K film you must go very slowly and be very careful not to pull to hard on any one bit of film since this film will tear very easily. Your fingers must be absolutely clean and glue free or the smallest bit of glue on them will start to tear the film. The same is true of all the other items that may touch the film. To help spread the film out evenly and to get the air out from under it you can blow very gently down onto the film, forcing the trapped air out. This job is finished by smoothing the film out with a very soft watercolor brush, or a mascara brush. If a drafting brush is used you must be careful to always draw the brush with the bristles sweeping back. If this type of brush is used with the bristles being pushed the ends of the hairs will tear many small holes into the film. Work with the film until it is well flattened and looks tight with no wrinkles. It is now ready to have the frame glued to it.
The glue that works best for attaching the film to the covering frame is the Prang Glue Pen from Office Depot. This is a very slow drying liquid glue that will allow a small amount of adjustment to the edges of the film to help you remove wrinkles. Glue stick can also be used if the area of the frame is not too large. Once the film is attached to the frame the whole thing is turned over and inspected for diagonal wrinkles or loose spots. Small placement corrections can be made by sliding the film on the still moist adhesive. The object is to get the film tight and wrinkle free on the frame before you use the adjustability of the frame to get the needed slack in the film. An inch of extra film on the outer side of the rails will make these placement corrections easier.
Now that you are ready to cover the model there are some things you can do to make the covering job better. If you are covering a very light structure like an EZB wing you will want to add a handle device to hold the wing spars straight and so that you will have something to hold onto as you place the wing into position on the film. This handle is constructed of a piece of 1/32" X 3/4" X 18" medium balsa with 1" balsa fingers spaced to hold the spar between the ribs. The ends of the fingers are glued with a very small dot of ambroid cement to the trailing edge spar. With six or seven fingers the spar will be very straight, The ribs will hold the front spar straight since the back spar is straight. The handle is glued with a slight droop, like the flaps on a conventional plane. This will allow you to easily place the wing onto the film with precision.
Now you place the wing onto the film without any adhesive to see if there is enough slack to allow the leading and trailing edges to meet the surface of the film. If it is too tight the wing will just rest on the top of the ribs. If it Is too loose the covering will have a great number of wrinkles, especially at the tips. Adjust the frame rails so that the entire wing rests on the film. It·is better to error on the side of having the film too loose. The wing is now turned over so that the glue can be applied to the top surface. Before the spray is sprayed you must cover the handle and fingers with drafting tape so that the spray is not applied to the handle.
The adhesive of choice is 3M #77 Sprayment, buy the larger can since it is much less expensive this way. This stuff is very sticky, do not use it near anything you do not want to stick things to. To apply the spray to the wing you need an area with a dark background and strong sidelighting. This will allow you to see the spray as it floats on the Holding the wing by its handle in your left hand, spray a small cloud of sprayment into the air and immediately pass the wing through the cloud so that the top of the wing picks up the spray. Repeat this step several times until the entire wing is thoroughly covered. You can test any questionable area with a small ball of scrap film. Just touch the ball of film onto any parts of the spars you think may not have enough spray on it. If the ball show any tackiness that area is OK. Once the wing has been sprayed take the drafting tape off of the model. Do this by holding the wing vertically by the handle and pull the tape down across itself. This will automatically pull off the drafting tape from the fingers as well.
It is best to get the wing onto the film fairly quickly once you have sprayed it with glue. Since you let only the very finest glue particles settle onto the structure this glue can dry out pretty fast. As long as you can get the wing onto the film in four or five minutes you will be fine. The covering should be done in an area with very good light so that you can tell just how to place the wing. Be very careful to put the wing onto the film right where you test fit it before gluing. Use the handle to get it just so before letting it touch the film. One good method is to rest the fingers of the handle on the rail nearest you with the wing tilted up and clear of the film. The handle is then slowly raise which lowers the wing down onto the film. Remember that you will not be able to easily lift it once it is down so position it carefully before you put it down onto the film. A possible error is to let a tip or one end of a spar touch in the wrong place and then try and force the wing back into place. If this happens you can lift the wing back off of the film by using a small clean brush with clean acetone to unglue the offending area. Placing the wing on the film is not that difficult, it just needs to be done carefully to get superior results, with the wing in place on the film you can gently push any parts that are not attached well down onto the film surface. Simply run your finger around the outline to make sure you are down everywhere. Look carefully and gently poke here and there until you are certain that you are attached everywhere. If there are any areas that you simply did not get any glue on you can take a solution of rubber cement thinned with the proper thinner and fix it. By using a very small, soft paintbrush you can apply a very small amount of glue to fix the offending area. The cement should be extremely thin, perhaps 10 to one or thinner. Any area where you had to use this extra glue you must allow to dry for a good while before the next step. If you do not need to add this glue anywhere you are ready to trim the wing.
A very low power soldering pencil is a good tool to use for trimming the new films. Do not use a medium or large iron as this will have too much heat coming off of the element. This can tightly shrink the film in very localized places and really mess things up. I use a 12-watt iron and often have to unplug it for moment or two to keep it from getting too hot. Also, never trim from under the film as the heat will rise and the film may collect hot air underneath until something goes bad. Trim from above and keep the iron moving. When the film is trimmed from around the wing several small tabs of film are left uncut to hold the wing in place. Once the majority of the film has been trimmed from the wing outline these tabs can be trimmed with the iron while the wing is held by the other hand. This way when the wing comes loose from the film it will not shift and reglue itself to the remaining film on the frame. Clean the tip of the iron frequently during your covering sessions to insure the very best cutting action.
Once you have the wing trimmed all the way around you can give the outline one last very careful inspection. You are looking for anyplace that the film is not attached. If you find any areas that need it you can use the thinned cement to fix them. Use this glue very sparingly. Allow the wing to dry completely so that if the wing touches some other film covered part in your model box it will not stick to it. The above method will work with stabs and other flight surfaces on typical indoor models. Most structures other than the EZB wing do not need the handle to straighten then, but it is a very good way to get any structure down onto the film with good control. I beats the method of "dropping" the structure onto the film and is very much better than trying to use your fingers.
If the wing fails to touch everywhere along the spar the film is too tight on the frame
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If the wing fails to touch everywhere along the spar the film is too tight on the frame |
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If the spars touch the film but the ribs do not, the film is too loose on the frame |
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When the spars and the ribs all rest on the film the slack is perfect |
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The handle is rocked on the edge of the frame to allow the wing to meet the film very evenly |
Larry Coslick's
Adjustable Film Frame
This view shows the arrangement of the turnbuckles that are used to distort the flexible frame and so adjust the film slack and remove any unwanted wrinkles.
Five turnbuckles are used, three crossing the frame to adjust the film slack and two diagonal to help remove any wrinkles.
Frame is built with enough flexibility to allow easy adjustment
The frame is made up of 1/8" X ½" basswood rails and ¼" X 3/8" basswood uprights and crossbraces. Size the frame to handle the largest model you expect to build. The film is attached across the top of the rails with liquid gluestick.
The following was not part of the original article, but is added by Tim Goldstein:
I personally do not use the technique of rolling the film into a ball as I find it easier to just roll the material out flat onto a anti-static mat and attach it straight to a frame from there. I was talking with Gene Joshu (the supplier of Y2K film) at a recent contest and on the newer Y2K2 film the crinckling technique is NOT recommended as the film is too likely to get holes from bubbles popping.
