Piper Navajo PA-31 Service Manual page 177

d. Cracks: (Refer to Figure 4-35.)
1. Before repairing a crack in the thermoplastic part, first determine what caused the crack and
alleviate that condition to prevent it recurring after the repair is made.
2. Drill small stop holes at each end of the crack.
3. If possible, a doubler plate should be bonded to the reverse side of the crack to provide extra
strength to the part.
4. The crack should be "V" grooved and filled with repair material, such as solvent cement,
hot melt adhesive, epoxy patching compound or hot air welded, whichever is preferred.
5. After the repair has cured, it may be sanded to match the surrounding surface finish.
e. Repairing Major Damage: (Larger than I inch in diameter.) (Refer to Figure 4-36.)
1. If possible a patch should be made of the same material and cut slightly larger than the
section being repaired.
2. When appearances are important, large holes, cracks, tears, etc. should be repaired by
cutting out the damaged area and replacing it with a piece of similar material.
3. When cutting away the damaged area, undercut the perimeter and maintain a smooth edge.
The patch and/or plug should also have a smooth edge to insure a good fit.
4. Coat the patch with solvent adhesive and firmly attach it over the damaged area.
5. Let the patch dry for approximately one hour before any additional work is performed.
6. The hole, etc. is then filled with the repair material. A slight overfilling of the repair material
is suggested to allow for sanding and finishing after the repair has cured. If patching compound is used the
repair should be made in layers. not exceeding a 1/2 inch in thickness at a time, thus allowing the
compound to cure and insuring a good solid buildup of successive layers as required.
f. Stress Lines: (Refer to Figure 4-37.)
1. Stress lines produce a whitened appearance in a localized area and generally emanate from
the severe bending or impacting of the material. (Refer to Figure 4-38.)
2. To restore the material to its original condition and color, uses a hot air gun or similar
heating device and carefully apply heat to the affected area. Do not overheat the material.
g. Painting the Repair:
1. An important factor in obtaining a quality paint finish is the proper preparation of the
repair and surrounding area before apply any paint.
2. It is recommended that parts be cleaned prior to painting with a commercial cleaner or a
solution made from one-fourth cup of detergent mixed with one gallon of water.
3. The paints used for coating thermoplastic can be either lacquers or enamels depending on
which is preferred by the repair facility or customer. (See NOTE.)
NOTE
It is extremely important that solvent formulations be considered
when selecting a paint, because not all lacquers or enamels can be
used satisfactorily on thermoplastics. Some solvents used in the
paints can significantly affect and degrade the plastic properties.
4. Another important matter to consider is that hard, brittle coatings that are usually best for
abrasion resistance should not be used in areas which incur high stress, flexing or impact. Such coatings may
crack, thus creating a weak area.
PIPER NAVAJO SERVICE MANUAL
Revised: 3/11/81
1H9
STRUCTURES