Epoxy 101

How it's used

We talked about “pure epoxy” in a previous post. It is now time to expand a bit on it, and find out just how it is used.

But first we are going to have to nail down a few key concepts, then we’ll start gluing up everything in sight.

If you remember, we decided to call pure epoxy a properly mixed amount of epoxy resin, and hardener, with nothing else added to it. 

But why, and what, would you want to add to pure epoxy?

The two most common epoxy additives are micro-balloons (aka “micro”), and flox (flocked cotton).

Micro is made up of microscopic glass balloons. This stuff is just plain weird! It flows almost like water, and it will fly into the air at the smallest upset. It is really important to wear a mask when handling it, or it could end up filling your lungs over time.

Micro is used in a variety of places, in three different consistencies: slurry, wet, and dry.

Micro-slurry is created by mixing equal parts of pure epoxy and micro (1/1), it is used primarily over bare foam to fill its pores and create a better surface for fiberglass to adhere to. 

Micro in any consistency, and slurry in particular, is not to be used anywhere near where a structural connection is to be made, because it creates a rather weak bond, good enough to hold foam to fiberglass, but not strong enough for anything else. In this case micro is acceptable because it is stronger than the foam itself, therefore the foam is still the weakest link.

Micro-slurry over foam (notice clear tape covering everything except the foam)

Slurry applied to foam with a squegee

By adding more micro-balloons to the slurry (2/1 to 4/1), we obtain a thicker material, with the consistency of honey, called “wet micro”. Wet micro is used primarily to join foam pieces together.

Adding even more micro-balloons (6/1 or more), we get a cake frosting type of material, called “dry micro”. 

Dry micro is used for three purposes: 

• Filling scratches and gouges on the surface of the foam, before it is glassed.

• Rounding inside corners before laying the fiberglass, to provide a smooth contour for the fiberglass to follow without lifting off the part (glass hates sharp corners).

• When very dry, as an external fiberglass coating, to smooth the surface before priming and painting.

Dry micro applied to a test piece

Dry micro sands very well with basic tools, and is used extensively during the finishing process.

Test piece sanded smooth and painted

Flox, on the other hand, is a structural material, and it is used in a dry consistency to make very strong bonds. The only disadvantage to flox, is that is is pretty heavy, and if overused it can add pounds to the airplane.

Sanding cured flox is hard, as is becomes tougher than concrete.


One of the most common uses of flox is in what is commonly referred to as a “floxed corner”, a triangular extrusion-like structure buried into the surface edge of a foam piece, providing a place for fiberglass to be joined together at an angle, usually 90 degrees or less. 

Because it would be impossible to structurally mate two sheets of fiberglass edge to edge, the foam is carved and the void filled with flox. 

Foam corner being removed

Any micro exposed underneath the cured fiberglass needs to be sanded away, and all the surfaces within the void wetted with pure epoxy, before floxing.

Foam corner being removed

Flox applied to missing foam corner

Two floxed corners

Care must be exercised in order to avoid any micro-slurry contamination of the flox/glass bond.

Fiberglass is then laid over it and wetted with pure epoxy.

Painting pure epoxy over cured flox

Fiberglass applied to two more sides

Finished cube