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How to Make a Fiberglass Cowling - If You Have To - Part 2

By Tony Bingelis (originally published in EAA Sport Aviation, April 1986)

Well, now that the cowling mold is completed, what next? Maybe we should do a little thinking about that before proceeding with anything else.

You are planning to make that cowling so that it can be separated and removed from the engine without having to pull the spinner and propeller, aren't you? As you can imagine, removing a constant speed propeller everytime you have to remove the cowling for engine maintenance could become an irksome chore. Even so, removing the spinner and a fixed pitch propeller is a big enough job for most of us. The conclusion is obvious. Split the cowling vertically or split it horizontally but split it you should.

Mold Preparation and Sealing
The first order of business is to mark, on the mold, where you intend to separate the cowling after the fiberglass lay-up has been completed. Use a black laundry marking pen or a similar type of felt marker. Don't worry, the drawn line will not be obscured by the fiberglass overlay and it will serve as an accurate useful guide for making that important cut.

Use a flexible straight edge and orient the parting line to roughly parallel the line of thrust . . . the top longeron if you have one. The cowl separation should split the air inlets approximately mid point to simplify installation and removal.

The next two steps are very important and must be completed before the fiberglass layup is undertaken:

  1. Seal the plaster mold.
  2. Wax the mold.

Unless you adequately seal the mold, the epoxy resin will penetrate its surfaces, and the fiberglass layup will stick to the mold making removal of the completed cowl (without the total destruction of the mold) almost impossible.

Of course, you would not ordinarily expect to salvage a mold built around an installed engine anyway. This method of fabricating a male mold is intended for use only in laying up a single cowling. Still, unless the mold is smoothly finished, sealed and heavily waxed, the inner surfaces of the cowl shell will be difficult to separate from the foam and plaster debris that will be sticking to it after you have broken it loose from the mold.

Commercial fiberglass fabricators generally prepare their molds by spraying them and building up a Cellulose Acetate Lacquer film over the mold. They follow that with two or three Carnauba Wax coatings . . . each rubbed to a high gloss. The final step is a spray coating of Polyvinyl Alcohol. The PVA (Polyvinyl Alcohol) is a sprayed on film in addition to the usual waxing. As an aid in the removal of the fiberglass layup, however, its value is debatable. PVA is a water soluble plastic substance used primarily as a parting agent between a fiberglass layup and a female mold.

To seal my mold, I used up my old leftover cans of spray paint and primer . . . spraying the mold with several coats to build up a smooth sandable surface. I was careful, however, not to mix enamels with lacquers as that could have caused a surface curdling problem wherever the enamel was oversprayed by a lacquer. Be sure to check the compatibility of the materials you use.

Taking a more studied approach to sealing the mold you could apply a heavy brush coat of polyurethane varnish directly to the plaster and, when dry, follow that with several spray coats of a quick drying sanding primer. Sand lightly after the second coat using wet/ dry paper (180 and 320), but be careful not to sand through the primer coats and into the underlying plaster. Any break-throughs should be repaired with several more localized coats of primer and sanded to blend in the patch.

After the mold is sealed and smoothed to your satisfaction, move on to the waxing preparation.

For my part, I find that three coats of a good paste wax, with each being rubbed to a high gloss, are sufficient to enable me to free the fiberglass layup from the mold easily enough. Use a wax containing a high percentage of Carnauba Wax in its formulation and remember to rub each coating down. Do this three times and your male mold will be ready to receive its first fiberglass laminate.

Fiberglass Selection and Handling
Any bidirectional fiberglass cloth sold by aircraft supply sources or those catering to amateur builders should be suitable to use for making your cowling. Purchasing the fiberglass cloth elsewhere (locally) might require more care on your part to insure that you are getting a cloth that will accept the resin and wet out properly. Glass cloth obtained from a boat shop will probably be O.K. but it may be that all they have is the heavier nine or ten ounce cloth. That would be all right to use, of course; however, the heavier the cloth, the coarser its woven texture and the greater the effort you will have to expend to finish its surface.

A six or eight ounce glass cloth is suitable for making all laminations. For the final lamination, however, I would apply a finer woven (four ounce or even a two ounce) fiberglass cloth to provide a smoother textured top surface that will require much less sanding to finish.

The woven fiberglass cloth, in its dry state, drapes quite well over curved surfaces and has enough diagonal play to permit you to spread it around most compound surfaces on the cowl. For highly curved compound surface areas use smaller pieces of glass. If the glass lays smoothly while dry, you can be assured that it will easily conform to the mold when wetted out with resin.

For forming the glass cloth around sharply contoured surfaces it is always helpful to cut the cloth pieces on a bias so that it can "stretch" around the curvature easier.

Keep your glass cloth clean. If possible, keep it away from the area where you will be using resin. You don't want resin to drip on it and ruin the cloth. If you allow dirt, oil or solvents to contaminate its surfaces, the resin will bond poorly and you may be troubled with delamination problems later. Don't lay the cloth on the floor to cut it unless the floor has been scrupulously vacuumed and cleaned, or overlain with a plastic covering.

It is much easier to handle the fiberglass cloth if you can spread it over a smooth slick surface . . . say, a table or a slab door.

Beginning with a large piece for the top of the cowling, roughly cut to size enough pieces to cover the entire cowling. Plan to butt the pieces in relatively flat areas but allow a few extra inches for overlapping the cloth on well rounded curves. Alternate the butt joints in each added layer so that no two butt joints are directly over each other.

Cut and number enough pieces to cover the cowling mold with a minimum of four laminations. Later, should you decide that some areas of the cowling are too flexible, you can always add another reinforcing lamination inside the cowling where needed. The air inlets and other well rounded areas will generally gain extra rigidity due to overlapping. This is O.K. as it provides the thicker buildup where it is most needed.

Don't forget to protect your aircraft structure from resin drips and runs. Do this by masking off the edges closest to your work zone. Grey duct tape and masking paper is fine for the job. While you are in a protective mood you might also spread a protective plastic cover over the floor under the cowl mold where you will be working. Rest assured the effort will not be wasted because that resin will be dripping all over the place.

The Fiberglass Layup
They say it is not the work, it is the preparation for it that eats up the time. Certainly that seems to be the case with fiberglass work.

Get started by mixing a small batch of resin . . . according to the printed instruction on the containers, naturally.

You can drape the dry glass cloth over the top of the cowl mold and pour the mixed resin on in the middle of the piece. Spread the resin with a brush or a squeegee, working from the center outward to the edges. I would suggest you brush on the resin before applying the glass cloth pieces that will be fitted to the sides. Applying the cloth to a resin wet surface helps stick it to the sides and bottom, but it can be a messy proposition especially when you try to butt-fit the adjoining pieces of fiberglass.

Work out any bubbles that appear with your paint brush and fingers . . . you are wearing gloves, aren't you?

Applying resin and glass cloth under the mold, all the while inhaling the fumes, is not exactly a fun thing to do so be careful. In addition, the resin will run down the brush handle and all over your hands if you allow it to do so. Take heart though . . . after the first layer of cloth is in place and wetted out, the additional laminations will stick quite well.

Build up layer after layer until you have the number of laminations you want. Unlike making a gas tank, your cowling need not be resin rich.

If you cannot make all the laminations during a single work session you will have to scuff sand the top layer to completely remove all the surface glaze. Should you fail to do that and add another lamination on top of the dry glassy surface it may not adhere properly and you may be faced with future delamination problems.

Here are a few additional tips. When making all the laminations during a single work session, you may find that you don't have to add any more resin to wet down the last layer of cloth. Simply use your brush in a stipling action and force the excess resin to the surface. As you lay the glass around the air inlet areas, do not drape the cloth ends too far inside the inlet opening of the mold. Remember, you will have to be able to pull the laminated cowl away from the mold. You can develop and finish those rounded openings (and the inside contours) later after the cowling has been separated and removed from the mold.

After the final layer of fiberglass has cured and hardened, use a power disc sander fitted with coarse sandpaper to level the overlaps and to finish shape the cowling. Since the cowling is not a structural unit don't feel guilty about cutting into the glass fibers. If you don't have to many overlapped areas you might prefer to do the final shaping by hand with a large bastard cut file. It would be less risky. In either case, rely on a chalk coated slat to help you find the high spots. Try to do most of this final shaping while the cowling is still on the mold. Once removed, it is hard to immobilize for heavy sanding.

It is well to realize that sanding cannot change the shape of the cowling. All it really can do is to improve its smoothness and gloss. That being the case, do all the shaping and fairing with the file or the power sander...  and don't get into a big hurry to switch to the finish wet/dry sandpaper stage.

When you are satisfied that the cowling is smooth and uniform, you have several finishing options.

  1. Apply a full brush coat of activated resin to the cowling to seal the exposed glass fibers, and to provide a sanding base for the final smoothing of the cowling. If necessary, immediately after applying the resin coat, cover the wet resin with pieces of waxed paper to keep the resin from drooping and running down the sides. Adding Cab-O-Sil to thicken the resin sometimes helps. Do not move or remove the waxed paper until the epoxy coating has cured. It will then peel off easily leaving a smooth slick surface.
  2. If you feel your cowl surface is still a bit rough and irregular because of a number of low spots and imperfections, you had better resort to applying a micro slurry to the cowling surface. A micro slurry is a mixture of micro balloons or "Q" cells and pre-mixed epoxy. The slurry is mixed thick enough that it does not run or sag. Apply the micro slurry to the areas of the cowling that need it and squeegee the mixture into the surface. After the micro slurry application has cured, sand the cowling smooth. If additional cosmetic work is needed, apply more micro slurry and sand some more. Often three applications may be necessary to correct serious defects. Once again check for high spots by rubbing your chalk coated smudge stick all over the cowling. Sand off the smudged areas and repeat as many times as necessary.
  3. If the initial surface of the cowling is fairly uniform and not too rough you could bring it up to a high degree of smoothness by spraying it with an automotive product called Feather-Fill. It should be sanded dry with wet/dry paper. Start with 180 and finish with 320. Feather-Fill is heavy so sand off most of it. This treatment should result in a beautiful glass-like surface.

Splitting the Cowl Shell
I usually do this the hard way with a hand held hacksaw blade, with the hand held end wrapped in a handkerchief. Using a saber saw would be much quicker but would raise all sorts of risks. Risks like sawing into the underlying engine or making the cut so irregular and wide that the cowling might be difficult to fit closely afterwards.

With the hand held saw blade, the cut can be made carefully and accurately along the drawn line made before the glass layup was begun. That line should still be visible through the layers of glass. Keep the hacksaw blade tilted at a shallow angle and the cut will be easy to make accurately and without the risk of sawing into the engine.

Liberating the Cowling
This could be an interesting experience. If you used polyester resin (I think epoxy cowlings are better) for your fiberglass layup, you might learn that removing the shell from the mold is a bit more difficult than if you had used epoxy. On curing, a polyester layup will shrink and a cowling (or a wingtip) laminated over a male mold will become difficult to liberate. Epoxy, on the other hand, does not suffer from this shrinkage problem.

To break the cowling shell away from the mold, I've had to use a tapered stick, small wood wedges, compressed air from an air hose, and even a short 2 x 4 and a rubber mallet to distribute impact blows selectively administered. Sometimes one or more of the techniques works quickly. Other times you have to persist to succeed in liberating the fiberglass shell from the mold. Eventually the separation is completed and there you are with two pieces of cowling laying on the floor.

Obviously by this time the mold is usually in a state of disrepair. But that's all right. The fun part is destroying that mold and unveiling the engine. Not so much fun, however, is cleaning up the debris.

For Your Good Health
When working with either epoxy or polyester resins, arrange for plenty of ventilation in your work area and abide by the warnings and instructions printed on the resin containers.

Epoxies are extremely toxic and even the so-called "Safe" epoxies continue to plague some builders with severe skin problems... apparently, regardless of the precautions they take. Don't assume that you are immune, or that you will continue to remain immune from the adverse effects of the resins or their catalysts and curing agents.

Polyester resin doesn't seem to cause reactions but the accidental spilling or splashing of the Catalyst MEKP that has to be added to polyester resins is a very serious hazard to skin and eyesight. Should MEKP splash on you or get into your eyes, immediately flush the skin or eyes with large quantities of water and promptly head for the nearest emergency clinic or doctor for treatment

When sanding anything - wood, resin, foam, plaster, paint - tiny particles fill the air around you. These particles can have a nasty effect on your lungs. Fiberglass dust, in particular, is composed of tiny razor sharp particles that can really do damage to your innards.

Protect your precious lungs by always wearing a respirator, or at least a good particulate mask, during any sanding activity no matter how short a duration that might be. It takes a lot of discipline to abide by that bit of advice, doesn't it? Just remember that dust masks are cheap while lungs are priceless. Many people now wear dust masks when mowing the grass and performing other dust raising chores. We are getting smarter.

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