The great thing about almost ready to fly airplanes is that for the most part, they come with great looking fiberglass engine cowls that fit beautifully and often are painted at the factory. The general method of attachment to the fuselage is the use of through-the-cowl screws making assembly quick and simple. After time however, as you build up flight time, engine vibration and everyday knocks and dings will take their toll and our once shiny engine cowl will start to show its age. Of course if one is available, you could simply replace your old engine cowl with a new one, but that’s going to cost you at least 20 bucks not including postage and handling. A great way to save that cash and apply it something you really need, like a gallon of fuel or a new servo, is to repair your cowl by yourself. Here’s how I did it for my Hangar 9 Piper Pawnee crop-duster ARF. It is a variation of the full-size repairs we made in the USAF when I worked on fiberglass radomes, flat panels and nose cones.
Here’s some of the basic tools and materials you’ll need to fix a cracked and worn-out engine cowl. You’ll need fiberglass cloth, Z-poxy Finishing resin, mixing cups and mixing sticks, masking tape, sandpaper, and a Dremel Moto-Tool with some Robart medium grit Carbide grinding bits. You’ll also need some tack cloths, a bottle of spray cleaner, acetone or MEK solvent, denatured or Rubbing alcohol, and some paper towels. You’ll also need a clean place to work and it helps to place some old newspaper or brown wrapping paper to protect your work surface.
Start by carefully removing the old engine cowl from your airplane. If some cracked or broken cowl parts fall away, be sure to save them. Set the rest of the model aside so you don’t get any epoxy resin on your model or the engine.
Thoroughly clean the engine cowl using some spray cleaner inside and out. To really degrease the surface of a very old and greasy fiberglass cowl use Acetone or MEK solvent. Use a paper towel and wipe the cowl down, then go over it again with the solvent to really get the oil and grime removed. If you don’t start with a squeaky-clean surface, your repair won’t adhere properly.
Inspect the cowl and indentify any cracks and damage you want to fix. The first thing you should do whenever you find a crack is to stop-drill it to stop it from spreading. Cracks are formed by stress and stress in a material always looks for the path of least resistance. Sharp corners and jagged holes and cutouts in cowls are where they pop up. When there is a sharp angle in an edge it causes a stress riser which concentrates the forces until a crack forms. When you drill a hole at the end of a crack, you show down its progression by spreading out the area the stress is acting on. So, whenever you see a crack, repair it as soon as possible so it won’t spread.
For some damage that happens from vibration and metal parts rubbing against the fiberglass (such as m
ufflers, ) the best way to repair and prevent the damage from spreading, is to simply use a grinding bit or cutoff wheel and enlarge the opening by removing the damaged portion. You can see this damage easily with its fractured gel coat and black grease deeply embedded in the edges of the fiberglass. After you degrease the area use a fine tip felt pen and mark the material you’ll remove.
Slowly grind away the material up to the pen mark then use some 220 grip sandpaper and smooth the edges of the opening so there are no more sharp edges to encourage further cracking. You can leave the edges as is, or you can lightly spray with a primer and then shoot a coat of matching color paint to seal the edges. This will help minimize future grim and oil from embedding itself into the fiberglass.
For holes that have cracked out to an edge, like where there attachment screws hold the cowl, you should use a cutoff wheel and lightly remove the very edges of the crack to expose new, clean material. On the inside surface use a grinding wheel to remove a thin portion of the fiberglass and then go over the area with some 220 grit sandpaper to expose clean material for the repair to stick to.
On the outer surface of the cowl, mask off the repair area so you can protect the undamaged finish. Use 220 grit sandpaper and go over the repair area and expose fresh gel coat and fiberglass. Wipe the inside and outside of the repair area and swipe it with a tack cloth to get all the ground fiberglass powder up and away.