Tech Tip

Forming a Composite Blister.


The restoration of a certain iconic homebuilt (the RV-1), required replacing a damaged belly pan, which in turn required duplicating the original fiberglass blister used to clear the base of the control stick. I have no idea how a skinny kid named Dick made the first one back in the 1960s, but it was my honor to make the second.

This part is different from most glass fabrications because the inside of the part is visible in the cockpit. We’ll take more care finishing the surface of the mold than we might for a part with one hidden face.

1 Start the male mold shape by cutting a top-view profile in a block of dense foam. My favorite foam source for this kind of thing is an ordinary 2-inch thick insulation panel sold at the big-box home-improvement store. The small-cell dense foam cuts and sands beautifully.

2 The finished part will need a flange at its base, so glue the foam block to a flat surface. I like MDO board (fine-grain particle board) for its flatness and smoothness. You’ll find it in the other aviation aisle at the big-box store, sometimes marked as shelf material. The quick adhesive for this task is a blob of cheap 5-minute epoxy. When cured, use a hand-held hacksaw blade to cut the side-view profile, and then bring out the sandpaper for shaping.

Rough it with 80 grit and tweak the final shape with 180 or 220. The board helps eyeball the shape, as it provides a reference plane. You can cut a template referenced to the board, if it helps. If you make a big mistake, just grab another chunk of foam and start over. Cost is low and it’s faster than any attempt at fixing it. Do what works, and remember the golden rule of moldmaking: Time spent on perfection now pays a large dividend later.

3 A filet is necessary between the base and the shape, as fiberglass cloth will not bend into a sharp concave corner. Mix an ounce or so of your favorite laminating epoxy with its hardener. Pour about half into a separate cup and mix in glass microballoons until the viscosity is like firm toothpaste. Pull on a latex or nitrile glove (You do have a box of gloves, right?) and simply use your fingertip to form an even filet around the base. Now look closely at your shape. Does it have any divots, dings, or flat spots? If so, wipe some micro mix into the bad place. Use a small squeegee; old hotel key cards work well and can be acquired from the desk clerk for the price of a smile. When you have things looking neat, with no excess micro, use a throwaway 1-inch chip brush to gently paint the surface with a coat of the plain epoxy you saved in the first mixing cup. Set the project aside in a warm place and go watch the ball game.

After cure, our male mold shape has a thin, hard epoxy shell. Gently sand with 220 grit in order to smooth the surface. You don’t want to break through the epoxy shell, but if you do it’s not a big deal. Finish sanding, paint on another coat of epoxy, and sand again tomorrow. The surface will only get better.

4 When you have it looking slick, spray on a few coats of high build primer from a spray can. You’ll find it in the bodywork supply aisle at the auto parts store. The epoxy shell will protect the foam from solvents. When dry, sand lightly. The primer will accent any remaining flaws. This is your last chance for a fine finish, so fix the flaws (if any) with a dab of spot putty, sand flush, re-spray, and scuff the primer again.

5 Satisfied? Shoot it with glossy paint. Anything on the shelf will do; we just want a high-gloss sealed surface. Moldmaking is done…isn’t it pretty?

6 First step in the layup process is applying a mold release wax.

There are waxes sold just for mold work, but the stuff you’ve been using on your car works, too. After waxing, use a cheap mini spray gun from the discount tool store to fog on a very light first coat of polyvinyl alcohol (PVA). Allow it to dry and fog on another. The third fog coat should cover nicely, without fisheyes from the wax. Spray a fourth heavy coat if you wish; it’s hard to add too much. PVA forms a water-soluble coating like cellophane or plastic wrap, which prevents bonding between the form and the new layup. As soon as it’s dry, you’re ready for glass fabric and epoxy.

7 The fabric selected for this application is 8.9-ounce 8-harness satin, sometimes identified as #7781. The loose 8-harness weave (7-over, 1-under) will smooth around the severe compound curves of our male mold, without cutting darts or overlaps, something that is hard to do with plain 1-over, 1-under weave. Each additional layer of fabric will build thickness at roughly 0.010-inch per ply. Four plies is plenty for a non-structural fairing. A strip of fiberglass tape thickens the flange where rivets will attach it to the belly pan.

8 When cured, gently pry up an edge and blast under it with an air hose. The part should pop right off the mold. Carefully trim the edge of the flange and sand smooth. Rinse off any PVA film with warm water. The extra care in mold shaping and surface finishing means this part looks as good on the inside as on the outside. Neither surface needs much additional work beyond normal prep for paint. No filler means a lighter part, always desirable, unless your homebuilt is powered by an anti-gravity device.

The epoxy shell, gloss finish and wax/PVA means you can use the form again, the bonus to good mold making. Who knows, this one may come out of retirement someday to make cowl bumps, or a streamlined cover for a GPS antenna.

Have fun!


  1. Very helpful tips. Been wanting to get into composites by starting with simpler shapes and parts such as this, and I see nothing here I can’t do. Helps build confidence that I’ll be able to do such things no problem, just need to start.

  2. Thank you gentlemen. Yes, dive in and make something. Aircraft Spruce sells a practice kit with a copy of Rutan’s “Moldless Composite Homebuilt Aircraft Construction”, a great reference. Buy it, build the examples, and you’re off to a good start.

  3. There is a guy on ebay that sells a few different aluminum blister shapes. I used one as a female mold and vacuum bagged it using my wife’s food sealer…. Came out great and was nearly paint ready out of the mold.

  4. The high build auto body primer sanded down to 80 grit and waxed plus the PVA spray can be the last step. Painting over that is a risk, since the paint could blister due to the exotherm of resin curing. The grey primer is heat resistant. From 70 years experience!

    • In this example, the epoxy resin seal coat was allowed to cure before light sanding and high build. No possibility of exotherm or topcoat blistering. However, it’s not strictly necessary to apply any paint at all; simply apply wax and PVA to the epoxy shell. In this case, high build and topcoat were used to maximize surface finish on the inside of the part. When pulled from the form, it was just as slick as the black paint.


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