This sprayer worked well for us but there are others available online or in your local hardware store that will work just as well! Open it up to reveal the mechanism inside...
Test fit your micro servo in a location where the movement can pull the can trigger. Affix with E6000 glue or epoxy and allow to dry (quadcopter foot being used in above photo as a spacer only while the glue dries).
Drill a small hole in the trigger for affixing a wire later.
Attach a wire to the horn of the motor and run it through the hole in the trigger. It's also useful to route out a bit of clearance in the handle itself for the wire to move.
Route the servo wire out the top of the can gun and leave slack on the trigger wire for calibrating later.
When prototyping, it's always helpful to have two of each part in case one breaks or needs troubleshooting! The can gun on the right suffered a small break while trimming the red trigger, which was fixed with pieces of a chopstick and copious glue. Learn from our mistake and only trim a small amount from the red trigger mechanism. =]
We also had success with this type of spray trigger, which was designed to attach to the end of a painting extension pole for spraying wasps' nests, etc. This type did not require any drilling, as the motor sits nicely at the pole junction, held in place with zip ties.
Holes do need to be drilled for mounting to the quadcopter, however. =]
To mount to IRIS+, we used a tomato cage (available at most big box home/garden stores), cut down to size. The round shape lets it fit against the bot's arms and attach with zip ties, and the three straight supports come down and stick into holes on the plastic sprayer.