I'm about to finish a 12wt rod (half completed many moons ago)
My first rod and a few guides I'd glued last time I worked on it went a bit barrel shaped- Thi ends and bulging in the middle.
What's the best way to avoid this?
Is it from too much glue or should I wait for it to firm up slightly?

Thanks.
I use a BBQ rotisserie motor to turn the rod

Too much epoxy combined with too fast rotation pronounces that effect. Your rotisserie way well be too slow though. As above light, multiple coats. The first coat leaves ridges around the thread when I apply it lightly. After application I let it rest without rotating for a minute or two, remove any saggy, drippy bits at the bottom then do what mark says and do lengthwise brushing to even out. In effect you are trying to remove as much of it as you can, it will retain what it needs.
What epoxy are you using?