My instructions call for using milled fiber with epoxy in some instances, and other, microballoons and epoxy in others. In what situations do you use one or the other?

Tim

What airplane?

If you have milled fiber I'd just use it. It makes the joint stronger, or get some Epo Grip Model Matrix, its got milled fiber in it already.

The milled fiber makes a very strong joint. This stuff is great for gluing bulkheads into a fiberglass fuselage. It makes a very strong joint, stronger than epoxy alone.

The micro-balloons are a light weight filler that add no strength. They would be used to create wing fillets and other shapes that don't require as much strength. They add lots of volume with little weight.

BTW, microballons, and unscented talcum powder are the same thing for vastly different prices

sorry thats so incorrect, micro baloons are fine glass particles formed into spheres, used for adding volume to adhesives and resin.

its used for increasing the volume of the adhesive to reduce the weight of the mixture without reducing the strength, caution should be used when sanding the adhesive/micro baloons, since the mico baloons sand into glass particles, a dust mask should be used at all times when handleing the stuff,before,durring, and after use, it doesn't break down when inhaled.

talc is finely ground gypsum, and absorbes the adhesive, adding volume without reducing the overall weight of the adhesive,it also adds no strength to the resin.

milled fiber is just stranded fiberglass which also absorbes resin adding strength but not reducing the weight of the mix like pilotfighter pointed out.