Flash only until ink does not pick up on your finger. The ink will be malleable that can be dented by your fingernail. The ink might be a little tacky. That is ok as long as ink does not pick up on your finger. One second too short of curing will pick up on your finger. One second too long and the ink is over cured. Precise control of exposure time is required, and can only be achieved with an automatic flash dryer.