FIBERGLASSING FOR MODELERS
(Part 3 -- Cont'd.)

Laying up a new cowl is fairly easy but some repairs appear to be much more difficult, especially if you can't work from both sides, so let's take a look at repairing fiberglass.

When you can reach both sides look at the damage closely and notice that the damage spreads out in a cone shape from the point of impact. This will take the form of cracked resin, fiber breakage, and delamination. After removing the gel coat or paint, examine the surrounding area and remove the damaged laminate with a high speed cutting tool.

DO NOT use a reciprocating saw as the rapid up and down motion will cause delamination at the cutting edge. High speed and slow feed is the best method to prevent delamination. Cut out the damaged area in a circular or oval pattern to prevent stress fractures at corners. Remember the Comet airliner! If you don't remember the Comet, it was a British jet airliner manufactured after WWII. The Brits used square corners in the windows in the passenger cabins and several went down in a short time. Investigations of the wreckage found fractures radiating outward from the window corners and were determined to be caused by metal fatigue, causing the planes to crash.

Try to match the strength of the original part by using the same number of layers and resin. When possible, increase the thickness of the part at the repair in order to gain stiffness, which increases at a factor of three. In other words, increasing the thickness by 25% increase the stiffness by almost 100%! Increasing the thickness 50% will make the part almost 3.4 times stiffer!

After "taper sanding" the repair edge keep it meticulously clean, using only solvents and clean rags. Cut replacement plies as close as possible to the original repair area. Make each succeeding ply slightly larger than the one before. If you can reach the rear of the damaged part, place a backing plate over the damaged area. Wet out each ply working from small diameter to larger diameter pieces taking care to match fiber orientation as much as possible.

Upon reaching the surface plies you may want to add a sacrificial ply for fairing purposes. Once cured, the area can be sanded fair for gel coat or paint application (be sure to test a gel coat over epoxy to check for compatibility). To blend the top coat into the undamaged area one must take care to allow for the thickness of the top coat(up to 20 mils for gel coat or 2-3 mils for linear polyester paints).

We hope that this article has convinced everybody that fiberglassing a model is not Rocket Science. If we can do it, anyone can. We will not discuss my first attempt but we will say that gathering all tools and materials before hand is paramount!

Next month we will discuss Polyester Plug and Mold Building....

Never use or try to repair a damaged propeller. You may get by with it a time or two, but is the cost of a propeller worth risking injury to yourself or a friend?

If the propeller is visibly damaged, then whatever force did that could also have caused other damage that remains invisible to the naked eye. So, please when you have a damaged propeller, either use it strictly for static display purposes only, or better yet, break it clean in half before discarding to keep anyone else from using it. Don't even think about using it as a back-up spare.

There are some solid black propellers on the market, which become invisible to the naked eye once they're spinning. This is a dangerous hazard which can be remedied by simply painting the propeller tips with a bright color. You can even use the paint to help balance the propeller. You do balance your propellers don't you?