I am an objects conservator with decades of experience in the use of Acryloid B-72.

Since the use of it depends on the evaporation of the solvent, which is usually acetone, the solution that you started with is constantly getting thicker and moving towards that solid state. 

First, in use, always dip your brush in, then close the the lid to the container. This keeps it from evaporating, it keeps you alive by not breathing the fumes for extended periods of time. When you are around acetone there are serious health affects - you need very good ventilation with either a fume hood or fans. Always get the MSDS (Material Safety Data Sheet) for every chemical and product that you have and use - it's required by OSHA. So dip and close is highly recommended.

Second, you should add a little acetone to your container each week, even if you are not using the solution, because over time even a bottle of B-72 on the shelf will thicken and harden. So by adding a little bit and stirring it with a wooden chopstick or glass or plastic rod, you keep the solution maintained. How much do you add? It depends how much you have and what size jar it is in. Go by consistency of the fluid. A 10% solution is usually used for labelling. That consistency will be pretty close to that of water, maybe a little thicker. So add a little acetone, stir, close the lid and shake, then dip your brush in and see if the solution drips off the same way it did when you first bought it. 

If you wait until the B-72 gets hard you pretty much will have to dispose of it and buy a new dilute solution. We in conservation make our own solutions of B-72, depending on whether it's for labelling, using it as a protective isolating layer, consolidating a very fragile object, or as an adhesive. But mixing those solutions means using a laboratory scale and graduated cylinders and lab beakers and a fune hood. So, if you don't have those and don't have a conservator nearby, then you'll have to buy the B-72 kit for labelling.

Don't substitute with nail polish. The reason B-72 is used is because it has been scientifically tested and it remains reversible for decades. It is a very stable resin that has wide uses in conservation for that reason.

Cheers!

Dave

David Harvey

Senior Conservator & Museum Consultant

Los Angeles CA USA