Those tips from John are very good. I have only rarely used the plastic bag method because it requires a relatively large mass of epoxy, and about half the uses, it overheats and "goes off." I usually use small batches and deliver the mix to the fillet area in dabs. Then I follow up with a double gloved finger or thumb to smooth things out. The mix being already spread out, it hardly ever goes exothermic and overheats. OTOH, if faced with a large or very long fillet, I would definitely use the bag technique, but would shift to a slower hardener to prevent the mix from going off. Most of the time, I am working in a colder space, 60 F or colder, and must use fast hardener.