Applying head cement “after” you have whipped finished your fly can be messing, especially when materials near the eye of the hook are feathers. A dab of head cement near the eye of the hook to lock in the thread can “smear” on other materials causing them to be hardened/cemented. You sure don’t want that to happen when using soft hackles near the eye.
I discovered through the “Intra-Web” a unique way to apply head cement. Do it before you do your whip finish. I use Sally Hansen’s Hard as Nails and just before I whip finish I’ll add a small dab of Hard as Nails to the thread. Just a small amount maybe an inch long. It soaks into the thread and when you complete your whip finish and cut your thread, the head cement is already applied. When it dries (which is very quickly) your thread is locked in without having to “dab” a gob of it near the eye. Keeps the eye of the hook clear of head cement too. Try it – you’ll like it!
