Covering the Aeromodel

Covering the Model

TWO COLOR COVERING

The introduction of heat-shrinkable plastic covering has saved a lot of labor and time for many modelers. Plastic coverings require far less effort and skill than the silk and dope or silkspan and dope coverings of the past. A tight, glossy, attractive covering job can be achieved by anyone with the right tools and a little patience.


As with any other covering methods, there are inherent drawbacks to plastic covering. One of the major problems is that of applying trim sheets to the completed covering surface. Several methods have been devised to overcome the problem of applying the trim sheets without having bubbles appear later. Some of them work far better than others but there is no assurance that bubbles will not eventually appear. This is especially true with large trim sheets. The larger the area, the greater the chance that trapped moisture will expand and cause bubbles.

Sometimes, the need arises for a multi-color covering pattern that requires large but relatively simple trim patterns. In this case, there is a viable alternative to using large pieces of trim. The pattern can be cut from the covering material and joined into a single piece before it is applied to the model. The method described is for a two (2) color pattern but can also be applied to more complex patterns such as the scallop, stripe, and sunray patterns used on a typical Citabria. It can even been used for camouflage patterns that have sharp edges between colors.

The process begins with deciding how the pattern is to be laid out to achieve the desired results. Illustration 1 shows a Thunder Tiger Trainer 40 that was recovered with red and white material to duplicate the original color scheme as closely as possible. 

The closed surfaces of the fuselage, stabilizer, and fin are covered using normal practices. The white of the fuselage is cut to the desired shape allowing a 1/4" overlap and applied. The red is then cut to shape and applied with the overlap.

The open bay of the wing requires a different process. There is no surface onto which the edge of the white covering can be sealed before the red is applied. In this case, the white and red pieces are joined together before they are applied to the wing. The bottom of the wing is solid white and is covered prior to beginning the layout of the top surface.

Templates for both the white and red patterns are cut from a heavy card stock to ensure that the patterns are the same on both upper wing surfaces. The template for the lighter color, white in this case, must be cut 1/2" larger to allow the darker, red, to be joined with an overlap. The overlap can be as little as 1/4" but this leaves no margin for error when applying heat to shrink the material. An additional 1" to 2" allowance is made on the outer edges to allow handling and pulling the finished material when it is being applied to the wing. This is normal practice for all plastic covering materials. The amount of this allowance is left up to the discretion of the builder. The templates are laid over the covering material and used to cut the outlines of the pieces that make up the final piece. Illustration 2 depicts the layout for the pieces of covering material used on the left wing of the Thunder Tiger Trainer 40.

The right wing panel covering sheet is made by flipping the templates over on the covering material and cutting the pieces exactly opposite of those for the left wing.

After the covering pieces are cut, they must be joined together before covering can begin. This is a critical stage. The joint must be strong enough to resist being pulled apart when the heat is applied to shrink the covering material. The backing material can be left in place on the lighter color material but part of the backing must be remove from the darker material to allow the joint to be made. This is accomplished by pulling the backing material loose along the edge that will be joined and cutting it with scissors roughly 1/2" to 1" from the edge. This allows room for working with the edge but the majority of the adhesive surface remains protected from contaminates.

A solvent such MonoKote Trim Solvent or acetone is used to activate (soften) the adhesive along the overlap. A soft brush, like a camel hair artist's brush, that is the width of the overlap is used to apply the solvent. The solvent must be applied evenly over the entire length of the overlap. After only a few seconds, the adhesive will be sufficiently tacky so the parts can be joined. The lighter covering should be held in place on a flat surface so that it will not move while the darker piece is being joined. Illustration 3 shows the light and dark pieces joined with a 1/2" overlap.

 Care must be taken to place the overlap joint at precisely the desired point with a minimum of movement required. Any movement can cause the adhesive to smear over the surface of the lighter covering. After the joint is made, it is pressed down with a squeegee to ensure that no air is trapped in the overlap. Although the solvent will evaporate very quickly, it should be allowed to sit overnight to be sure that it has adequate time to "gas out". 

After the covering sheets are made, they can be used to cover the wing panel using normal covering practices with one exception. The corners are tacked down with a covering iron then the edges are pulled into place and tacked down. Illustration 4 shows the finished covering sheet laid over the wing panel.

When heat is applied to shrink the covering, extreme care must be take to avoid over-exposing the edge of the darker material to the heat. Although the edge is joined, it is still a raw edge and is subject to pulling back. The solvent welded joint should hold up to normal shrinking without pulling apart at the seam.

After the covering is applied, it is trimmed along the outer edges to complete the process. To further accentuate the color scheme and to help protect the raw edge, pin-stripe tape can be applied over the raw edge at the seam. Illustration 5 shows the left wing panel completed. If the wing is one piece, the covering of the opposite wing panel is allowed to overlap at the center of the wing.

With proper planning, almost any multi-color trim scheme can be applied in the same was as the two color scheme described. Using this method is more work but the advantages far outweigh the disadvantages. There is a slight reduction in the weight of the covering but this is relatively insignificant. There is far less chance of bubbles appearing. Reheating to tighten the covering results should it become loose has much better results. Builders who have had problems before that his method virtually eliminates those problems.

SILKSPAN COVERING

Over the last four (4) decades, the choices of covering materials for models have expanded steadily and R/C modelers have quickly adopted the new methods. Modelers involved in other disciplines, especially control-line stunt, are not as eager to change their methods of finishing models. There must be a reason for this. The primary reasons are that it is easy, cheap, fun, and above all, beautiful. Silkspan, the primary covering material, is lightweight, accepts nearly all paints readily, and will never sag, bubble or wrinkle. It goes on just as easily over either sheeted structures or open framework.

Silkspan is primarily used on smaller models like Old Timers, 1/2A glow and small electrics, but it is also an excellent surface preparation for sheeted surfaces even on giant scale warbirds. There are two (2) primary disadvantages to using silkspan; it is easier to tear or puncture than plastic coverings and requires much more time and effort to finish.

R/C modelers could learn a few things about finishing their airplanes from control-line modelers. These people can make the most phenomenal finishes and keep them light enough so that the model is competitive. Maybe this is in part the reason that control-line stunt models are scored on appearance and R/C pattern planes are not.

The stunt community in general frowns upon anything that irons on. The purpose of covering the balsa structure with silkspan is to hide the grain, not fill it, and to add strength. It makes a tremendous difference in strength. According to Windy Urtknowski, a guru of stunt model finishing, there is no way to fill balsa grain at an effective weight. He says he has tried covering with glue and sanding it off, but that the grain reveals again after sitting in the sun for a while.


The steps required to achieve that fabulous finish are:

1.     Sand everything as smooth as possible with 400-grit paper.

2.     Brush 3 coats of clear nitrate dope thinned as little as possible but still resulting in good paint flow. These coats must provide a reasonably waterproof seal so that when the wet silkspan is applied, the underlying structure will not warp due to the moisture.

3.     Again, sand everything as smooth as possible with 400-grit paper.

4.     Start the covering with the bottom of the wing. Lay the wing on a clean work surface and trim a sheet of silkspan to oversize allowing 1" to 2" of excess around the perimeter. Wet the silkspan with water until it is completely saturated. This will cause it to swell and wrinkle.

5.     Gently lay the silkspan sheet over the surface to be covered. Start lifting and smoothing the silkspan until all wrinkles are removed and it is pulled fairly taut. Use wet brush to help to force bubbles toward the edges being careful not to tear the silkspan. Even wet, it is surprisingly tough.

6.     Using a sheet 240-grit paper, sand the edges on the down-stroke only to feather away the excess silkspan. The silkspan can be easily worked around compound curves, leading edges and wingtips.

7.     Once the silkspan is trimmed and while it is still damp, brush on a coat of nitrate dope that is thinned 50%. The dope is this highly thinned so that it will partially dissolve the dope that is already on the bare balsa. This will bond the covering to the airframe.

8.     Cover the rest of the wing and then the fuselage following steps 4 thru 7. Overlap the successive pieces so that there are no gaps.

9.     Brush 3 more coats of clear.

10.           Sand everything as smooth as possible with 400-grit paper.

11.           Make a sanding sealer using equal parts of thinner, clear dope, and corn starch or unscented talcum powder.

12.           Brush on a thin coat of the sanding sealer over any remaining pits or dings.

13.           Sand off as much sanding sealer as possible with 400-grit paper.

14.           Sand everything as smooth as possible with 400-grit paper.

15.           Spray a coat or two of 50/50 clear/thinner to seal the filler coats.

16.           Sand this lightly with 600-grit paper.

The ultimate goal of this process is to make all of the surfaces as flat as possible then use the dope and silkspan to make it smooth. The trick up to this point is to use as little thinner as possible in the mix. Thinner changes the shape of the wood taking away from the flatness of the wood and requiring more sanding. The trick to finding flaws is to sand in a room with only one light source. Hold the model up to the light and bounce the light off the working surface on at an oblique angle. This will make even he slightest flaw visible. This technique is called candling. All flaws must be corrected at this point; otherwise they will be even more visible after the color coat is applied. Silver primer is very important to an award winning finish, especially when translucent paints, such as candy apple automotive paints, are used.

17.           Spray a coat of silver dope. Allow it to dry for about a week. The longer it dries, the easier it is to sand to a smooth finish. If the paint balls up while being sanded, it did not time to dry sufficiently. Stop immediately and allow a few more days for it to dry.

Normally, the reaction when the silver is sanded will be frustration. Every flaw is highlighted. Sand off as much of this silver as possible and correct all flaws with sanding sealer.

18.           Apply a second coat of silver.

19.           Again, correct all the flaws.

20.           Continue this process on the surface has the appearance of machined aluminum. The silver dope is actually used ultra fine filler coat.

Note: Do not spray different colors over each other. This adds weight and makes the color harder to apply. For example, do not paint the entire surface white and put blue trim over the white parts. Mask the areas that are to be painted white and spray it. Remove the masking and allow this coat to dry thoroughly. Mask over the white and spray the blue trim. The silver dope is a perfect base for all colors and saves weight. Finishing this way takes a lot of work and time but the results are incredible.

21.           Mask and spray the lightest color paint first.

22.           Mask and spray the remaining colors.

23.           Sand the color coats to a dull finish with 1200-grit paper, paying close attention to smoothing the edges as much as possible without through the color coats.

24.           After an even, smooth surface is achieved, spray about 4-6 coats of clear over the entire surface.

25.           Let this cure at least a month. The harder the paint, the shinier it will be and the longer it will keep its shine.

26.           Rub the whole plane with fine rubbing compound.

27.           Rub the whole plane with Gorham's Silver Polish.

28.           Wax it 2-3 coats with fine automotive wax.

This covering technique is lighter than most of the plastic film coverings, will never wrinkle, and is quite easy although time consuming to do. The results that can be achieved from this method are incredible and unattainable with film coverings.