When i 1st started i was di only, and as such, i sometimes found myself having to change resin when out. I soon got into the habit of, if it happened, finding somewhere with a rough tarred, or concrete surface. This provided the necessary grip to the plastic base of the bottle, whilst i leaned down hard and twisted. I also, through trial and error, came to the conclusion that the top was easier to come off once the pressure had been released.