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Make a Fiberglass Cowl

May 03, 2015

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Airfield Models ( to Make a Fiberglass Cowl

I have received this question so many times that I decided to go ahead and write an article even though I don't have photos to illustrate at this time.

This tutorial covers how to make a one-off cowl or other fiberglass part.  The mold is destroyed in the process.  If you need to make multiple parts from the same mold then this is not the way to do it.

If you were to need a bunch of the same part, then you’d need to make a permanent mold which is expensive, time-consuming and not covered in this article because I don't know how to do it.


Items Needed to Make the Cowl

You will need the following tools and supplies

  • Green Florist's Foam — This will be the mold.  You may have to join several blocks together to make a piece that is large enough.

  • Spray Glue — Used to join blocks and attach them to the firewall.  I join blocks with spray glue because it is not significantly harder than the foam when it's dry.  However, it dries rubbery and doesn't sand well.  In the end, that doesn't really matter.

  • Wide Masking Tape

  • Automotive Paste Wax

  • Coping Saw

  • Carving Knife

  • Sanding Blocks

  • Fiberglass Cloth — I generally use both 2 oz and 0.5 ounce cloth.

  • Finishing Resin — I use slow drying epoxy finishing resin.  Polyester resin can also be used but it is more brittle.

  • Squeegees and Disposable Paint Brushes


Making the Cowl

Before you make the cowl, the front end of the fuselage should be complete.  All sheeting must be in place and it should be sanded to shape.

Do not finish sand because the fuselage will need to be sanded after the cowl is made.

  • Join the blocks to make one larger block that is big enough to make the cowl.

  • Sand the back flat.

  • Draw centerlines on the top, sides and front of the cowl to use for reference.  A Sharpie fine point marker works well.

  • Place the block against the firewall and draw the outline of the fuselage on the back of the block.

  • Draw the outline of the top view on top of the cowl aligning it with the outline drawn on the back of the cowl.  Use the coping saw to cut the top view outline leaving it slightly oversize.  Draw new centerlines to replace the ones you just carved away.

  • Now draw the side view on the side of the cowl.  Cut the side view, again leaving it oversize.  Replace the centerlines.

  • Cover the entire firewall with masking tape.  I apply the tape so that the edges butt together but do not overlap.

  • Wrap 2-3 layers around the forward fuselage from the firewall to about 4" behind the firewall.  This tape should overlap itself.  It should also overlap the firewall by about 1/4" to ensure that any resin that gets between the foam and the firewall doesn't glue the cowl permanently to the fuselage.

  • Apply a few good coats of auto wax to only the masking tape wrapped around the fuselage.  Do not apply wax to the tape on the firewall except for a small bit around the outer edge.

    Keep wax away from bare wood or it will cause you a lot of headaches when it comes time to apply the finish.  This is why you put tape so far back.

  • Wrap newspaper around the front of the fuselage to keep spray glue from getting on the wood and the waxed tape.

  • Apply spray glue to the masking tape on the firewall and to the back of the foam block.

  • When the spray glue is tacky attach the cowl using firm pressure.  Give it about 15 minutes to set up.  Ensure that it is firmly adhered.  If the foam does not feel like it will stay in place remove it and try again.

  • Carve and sand the cowl to finished shape.  The foam will make a huge mess so I suggest you do this outdoors.  Take your shop vac with you so you can vacuum the dust from you and not carry it all back inside.

  • Cut a piece of 2 ounce cloth to cover the front of the cowl.  It should overlap the top and sides approximately 1".

  • Cut a piece of cloth to wrap around the cowl.  It should overlap onto the fuselage about 1" and the front of the cowl by the same amount.

  • Vacuum the cowl and fuselage.

  • Mix up the resin according to the directions.  Put the piece of cloth on the front of the cowl and brush resin into the cloth.

  • Immediately apply the piece of cloth around the cowl using the same batch of resin.

  • Allow this to set up until it can be sanded.  Lightly sand the cloth to remove crud and to feather the overlapping cloth.  Do not attempt to sand the cowl smooth at this point and do not cut into the cloth.

  • Continue adding layers of cloth until the cowl is as strong as it needs to be.  I find that 3-6 layers is usually plenty.  In fact, I almost never use more than 3 layers.  A fiberglass part can be very flexible and still be strong enough.

  • At this point I sand the cowl as smooth as I can get it and add a final layer of 0.5 ounce cloth.  I think it makes the cowl easier to finish, but it's not absolutely necessary.  You can brush on a final coat of resin instead.

  • Allow the part to cure for another day or two.  Longer is better.

  • Now the cowl needs to be broken loose from the fuselage.  I grab the front of the cowl and fairly violently push, pull and tug it until it pops loose.

  • While the foam is still inside, I make the cut-outs in the front of the cowl.  I follow this with more dry sanding and finally a good wet sanding working through various grits.  If you go through the cloth, apply more resin to those areas and sand some more.

  • Now cut the back the of the cowl so that it overlaps on to the fuselage by however much you want it to.  I normally allow about 1/2" of overlap.

  • Gouge the foam from the cowl.  Use coarse sandpaper to remove all the foam adhered to the fiberglass.  Some people use solvent or gasoline, but I think this is a bad idea and it may damage the cowl.  Some solvents can break down epoxy.

If you feel the cowl isn't strong enough after you remove the foam, you can add more layers of cloth with the cowl detached from the fuselage.

Again, the cowl does not need to be very thick or very stiff.  If it is mounted properly it will last as long as the rest of the model.

At this point you can mount the cowl, make whatever cut-outs are necessary and finish it as you would any other fiberglass part.

Mike James of NextCraft has some good composite techniques on his site.  I believe he has some photos and a how-to that are similar to what I just described.


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How to Align Model Aircraft Cabane Struts
How to Make Cowling Cut-outs

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Copyright © 2004 Paul K. Johnson