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.

Illustration 1

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.

Illustration 2

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.

Illustration 3

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.

Illustration 4

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.

Illustration 5

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.

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