REPAIR OF ORIGINAL GRUMMAN PLASTIC PANELS,
WHEEL PANTS, ETC
By: Bob Mears,Cheetah N9813U

Let's begin by stating that the plastic used in the aircraft panels and
wheelpants is ABS (a styrene-based plastic). It is not Fiberglas which is a
polyester resin and it is not PVC which is a polyvinyl plastic. Some folks
have used general purpose PVC pipe dope and have indicated they have had good
results. I have preferred to stay with straight ABS since I want a fusion of
the parts with the same base material. This is not a gluing operation. Gluing
normally provides a mechanical bond rather than a chemical one. This process
actually welds (fuses) the part back together and fills the crack part with
the same base material.

Also, it should be known that the plastic parts on your aircraft are
vacuum-formed from flat sheets heated to a temperature near their melting
point. By their nature these parts remember they were at one time a flat
sheet (called plastic memory.) When they were formed into these complicated
parts, stresses were frozen in as the parts cooled and they really want to be
flat again. You can prove it to yourself by heating the parts above a
softening temperature and watch what they do. As the parts age, they become
more brittle through molecular oxidation and degradation. The cracks you see
are the results of the stresses being released at the weakest spot when the
strength of the part can no longer hold them in. Once relieved, the part
usually quits cracking unless screws, etc place external stresses on the
part.So, the intention of this repair is not to try and bring the parts back
together but rather to fill in the cracks and halt the continuing cracking of
the part at that location. The stresses have already been relieved when it
cracked so the tendency to crack at that location is greatly diminished.

That should be enough of a lesson in plastics. <g>

You will need some materials and tools for this work so heret hey are:

An active ABS solvent which can be found at your hobby store. I found mine
at Hobby Lobby. It is Methylene Chloride, a clear solvent used for assembling
styrene models such as cars, planes, etc. and comes in a small glass
container with a applicator in the cap of the container. Get at least two
containers while you are at it.

Find an old discarded plastic panel from a printer or computer cabinet. These
cabinets are made from ABS and there are always parts that come with parts
you never use. If your aircraft panel is tan, the typical tan computer
cabinet parts are very close in color.

Xacto knife

220grit sandpaper. Other grits should be ok but the courser grits make a lot
of marks. It should not be too fine a grit because you will use them to
remove surface grit and renew the surface for repairs.

If you are working on wheel pants, consider having a Dremel tool available.

Fiberglas mesh

Preparing the ABS paste:
Cut small pieces from the old computer cabinet part and put them in one of
the bottles of solvent. Continue to add the pieces until the mixture becomes
a medium-thick paste. You should be able to spread on easily and it should
flow into the cracks of the aircraft panel. REMEMBER: the solvent is very
volatile so keep the top with the applicator on the bottle at all times when
not in use. If you don't, the paste will thicken quickly and will become
unusable. If the paste does thicken, you can add solvent to the paste to keep
it at the consistency you want. Also, have good ventilation when you use the
solvent.

Keep the second bottle as pure solvent.

Repairing thePlastic Panel:
[Reminder: you will be working with the inside surface of the panel which
usually is clean except for dust,etc. Don't let the solvent contact the face
of the panel anymore than necessary since it will disturb the surface. ]

Clean the inside panel surface with soap and water and dry thoroughly.

Apply the solvent from the back side. That way, you will minimize any damage
to the appearance of the face of the panel. Also, the panel is very thin so
handle it carefully so as not to further damage it.

Wipe the crack out with the pure solvent. It will soften the material and
prepare it to accept the paste. Make certain the solvent wets all of the
crack. Immediately put the cap with applicator back on the bottle when it is
not being used.
While the surface is soft, apply a generous helping of the paste in the
crackand spread it to the surrounding surface. ( Cover at least 3/8 inch
around the crack).

Immediately place a piece of Fiberglas mesh of similar size over the crack
and entangle the webbing in the ABS paste. You may want to brush some fresh
paste and/or solvent on top of the mesh so that it becomes an integral part
of the mix. You don't want to build up too much thickness since it has to
fit back at its original location on the plane. This is particularly true
around screw holes which generally have multiple cracks.

Let the panel sit undisturbed overnight while the solvent evaporates.

Remember, the part is generally somewhat more brittle than a new part so
treat the repaired part with some care otherwise you could start the cracking
problem up all over again in another location.
If you want to make it look like new, go to your nearest DuPont auto paint
store and purchase an aerosol can of SEM Phantom White #15003 tan paint for
plastic parts. Spray the surface of the panel and it will look like new. Be
sure to clean the surface before you spray it.

Repairing Wheel Pants

Start the process the same way except this time you will need to clean all
that crud out of the wheel pant and spend a lot more time cleaning the
surface to be fused. Also, the crack itself will be contaminated with grease,
dirt and no telling what else. You will need to be certain it is clean.

Sand the area around the crack on inside surface of the wheel pant until it
has that flat non-glossy appearance. Use your dremel tool or knife and
slightly "V" out the inside surface of the crack all the way to the tip of
the crack. You can even stop drill the crack if you choose to.

Apply the pure solvent with the applicator to the crack and surrounding area
until it is softened. If you plan to paint the wheel pant it will not matter
if the solvent finds it way to the outer surface.You can sand it down later
with a block sander.

Apply the paste until it overfills the crack remembering the paste will
shrink as it the solvent evaporates. It will probably be necessary for you to
repeat application of the paste at least twice to fill the crack. Just let it
sit overnight after the first application and then make the second one.

During the second application, press a similar size patch of the fiberglas
mesh into the paste so that it locks in place. Then apply more paste and/or
solvent until the mesh is firmly in place. If you desire, you can substitute
a flat piece of the ABS part for the fiberglas mesh. Fuse it over the crack
inside the wheel pant. It will reinforce the area around the crack. If you
look inside the wheel pant, you will see that manufacturer of the original
part used the same technique. Now let it sit up overnight until the solvent
has a chance to evaporate.[Remember: Just because the surface feels hard does
not mean all the solvent is gone.]

Replacing missing pieces of plastic.
I had a piece of my wheel pant missing along the wheel opening. So, I cut
out a piece of the computer cabinet to the same shape as the missing part. I
used the solvent to soften the exposed edges of the wheel pant and the new
part and fused it in place with the solvent and paste. You can fill any
holes or uneven places with the paste and sand it to match the contour. My
wheel pant has never cracked out again in the same place. So, you can be
creative with these repairs if you choose. You can do the same type repair
with the thin panels but I don't know where you would find a thin part with
the same surface finish. You could use parts from an old panel if you have
one sitting around somewhere. You can use a piece of the thin material on the
back side of the repaired panel to reinforce it if you choose. Painting the
parts generally hides any small imperfections. Paint the parts and the
aircraft will look like new inside.

And have fun.
Bob Mears