Algi-Bond softens the top 1/2 millimeter of set alginate so a new layer of alginate can stick to it. Terrific to have on hand when you are doing a very large job- just in case you run out of time and your alginate sets too quickly.
Just wet the surface of the "already set' alginate and wait about 2 minutes before applying the new layer. It will stick just fine.
This bag of powder is enough to make about 2 gallons of solution- which should last a very long time. Neither the powder or the solution have a shelf life so it should remain usable for a long time as well. Mixing instructions are on the label.
It won't hurt your skin, but this stuff should NOT be put into your eyes. Use only with adult supervision.