Robert (MP&C) here uses a brush for epoxy primer for small spots. You could even use a throwaway brush, mixing just what's needed for the painting, maybe waiting till the end of the day and brush primer all the spots you've been working on at the same time. The unmixed components should last a long time (nearly as long as an unopened can).
For what it's worth, I got a couple of spray cans of 2 component epoxy ($20 each) to try that out. We'll see how it goes. Supposed to last up to 2 days after activation.
