From my experience never use droplets to measure your paint especially the plastic one try to use a small stick or a brush to measure it than using this droplets. those transparent plastic one are imposible to clean and also after several time use expose to thinner intend to become unusable already. Mostly I use this for measure thinner only and for more durable type try find the glass one which is last way longer than this one.

1. U can hit the bottle cover, it might make the dried n hardened paint stuck in between the cap n bottle crack, so can reopen again.
2. Terbalik the bottle, put a little thinner at the edge of the cap. Try open it again. A little messy though.