Depends very much on the size of the surface you are gluing. For small structures I use toothpicks to apply mixed epoxy. You can visit a $1 store or such and usually find packs of TPs for 2 for a dollar, each with 100 TPs. Last a long time.

For large glue surfaces such as fuel proofing a nose/tank section use a small brush (again $ store stuff) and apply heat with a heat gun. The epoxy will run gin-thin and can be brushed well into the surface. Watch for a quick setting.

For general structure work, I strongly advise you use the cream-colored glues such as those found in any store selling building materials, or crafts. While working with these glues you may have to pin or clamp joints. A TOUCH of CA will hold stuff in place while main glue dries, so another option. For me, being a builder since the late '40s, I use whatever I think the structure needs.

Back to epoxy, if it is a fair sized joint like an ARF's wing tail joints, I use a pop-cycle stick, or a scrap piece of wood, or cardboard, or whatever me lil' ol' fingers can reach. Some make mountains from mole-hills.