Materials Used

Fiberglass aircraft are made primarily of foam, fiberglass, and epoxy.


There’s a number of epoxy systems you can use with your aircraft. This is a link to presentation on the Dos and Don’ts of epoxy

I elected to go with the MGS 335 system, which seems to be fairly popular amongst the different builders.

The MGS 335 system should be mixed at a 100:38 ratio of resin:hardener by weight.


Microballoons (or microspheres) are used with already-mixed epoxy to create slurry, wet micro, and dry micro. Microballoons should be stored in a covered container with as low humidity as possible. I use a small dehumidifier to help with this. To dry out microballoons, bake them at 250° Fahrenheit, then sift them with a flour sifter to remove lumps.

As noted, you mix microballoons with epoxy in the following mixtures, by volume:

MaterialMicroballoons : Mixed Epoxy mixture, by volume
Wet Micro2-4:1 (sags or runs like thick honey)
Dry Micro~5:1 (enough microballoons to create a paste that does not sag or run)

Though, really, you’re supposed to add thee microballoons until the desired consistency is achieved.

Slurry/Micro Slurry

Slurry is mostly used to paint or squeegee over foams immediately before glass cloth is applied over them. Do not let the slurry dry before you apply the glass cloth. With urethane foam, use a full thick coat of slurry.

Wet Micro

Wet Micro is used to join foam blocks.

Dry Micro

Dry Micro is used to fill low spots and voids.


Flox is a mixture of epoxy and flocked cotton. There are two types of flox: standard and “wet”. Standard is epoxy mixed with just enough flocked cotton to make the mixture stand up, whereas wet flox uses less flocked cotton, and is mixed so that it’ll sag or run, similar to wet micro.

When using flox to bond a metal part, be sure to sand the metal dull with 220-grit sandpaper. Also paint pure epoxy (no flox) on the metal part prior to bonding with flox.

Peel Ply

Peel Ply is a term for a layer of fabric used to absorb extra epoxy, smooth out the transition between extra layers of fiberglass, and to improve adhesion when applying additional layups of fiberglass to an already-cured layer. Peel ply is always added as the last layer in a layup, and is removed in the “green stage” - when the epoxy becomes more viscous and changes to being a very tacky kind of goop.

The FAA has studied this, producing a document The Effect of Peel-Ply Surface Preparation Variables on Bond Quality. Which concludes that a thick-weave, uncoated polyester fabric is the best fabric to use as peel ply. I use a white, presumably uncoated, thick-weave polyester sourced from a local fabric store.

Last updated: 2022-09-09 14:09:51 -0700