By JCAD - Inc.

January 14, 2018 at 6:52 am

Everyone has their own preferred method of building, as there are a lot of different ways of doing the same thing. However, in order to make a foam mold for fiberglass, I prefer to use the lost foam technique. This technique has been in existence for quite a long time and is widely used by most professionals.

Basically, with the lost foam technique, all you have to do is to dissolve a part of the foam to maintain rigidity, leaving you with the remaining part of the foam when the piece is done. To clarify, what you do is; carve the foam into the desired shape, cover the carved foam with layers of epoxy and fiberglass, and then discard the foam.

This method is commonly used for nacelles and fuse glass but you can also create a hollow composite wing with this technique. Check out this detailed but concise guide on how to make a foam mold for fiberglass.

Mold making and fiberglass layup

Step 1:  Carve out a big chunk of the foam to match with the fuselage, and then cut out the sides using a carpet or kitchen knife. Rough cut the sides with the knife to complete the shape and sand to achieve the perfect shape. This step can be adequately done in an hour or half.

Step 2: Once you’re satisfied with the shaping and final outline, you can wrap up the plug with a layer of packing tape to stop the epoxy from seeping into the foam. You can apply the wax to ease removal of the tape later on.

Step 3: Rotate the core so that you could work on all sides effectively. You can do this by placing it on a fixture with a nail holding it in place at each end.

Step 4: Apply a layer of fiberglass cloth, use 3M#77 spray before applying the epoxy. Apply the cloth in about 3” overlapping strips to ease the placement. You can mix your epoxy by using the manufacturer’s directions as a guide.

Step 5: Apply the amount of layers you need to get the desired rigidity and allow the epoxy to harden for about 24 hours or more.

Finishing processes

Step 1: When the epoxy is all dried, you can sand off the bad spots and rough up the entire surface.

Step 2: Mix a blend of microballoons and epoxy and brush the entire fuse. Let it harden to a point that it can be perfectly sanded.

Step 3: By using your sandpaper, sand off all the mixture except those present in the hollows until you get a smooth surface.

Removal of foam

Step 1: Remove the foam after you’re done with the sanding as it provides a strong base to apply pressure. You can use acetone or any other suitable chemical to remove the solvent, which will pour out freely once you open an area of the fuselage.

Step 2: Remove the packing tape by pulling a corner of the wax from the wing bay located inside. This will leave you with a clean interior as the wax will also remove the remaining foam paste that failed to pour out.


Spray with Kylon primer or a similar product of your desired color. Be sure to continuously wet sand the surface until you achieve the desired smoothness.

Quick tip: In order to increase accuracy, you can make use of cardboard templates and remember that fiberglass is a lot weaker on sharp corners. Therefore, consider adding more paste to help you sand all the sharp corners in the end. Making a foam mold from fiberglass is an easy process, but it requires a high level of patience and consistency.

Applications of this process

For one cool example of how people are applying these and similar processes, check out this book:


If this sounds too complicated to you, consider opting for a foam injection molding technique. It ensures top quality and you can easily outsource the whole process to experts without breaking the bank. We can help you out with that, ask us for a quote on your project, or email is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to If you use affiliate links in this page I will receive a commission.

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