Making a mistake with casting resin is a common occurrence while making jewellery. There is only one solvent available called Epoxsol that will dissolve casting resin. The solvent may seem dangerous and hard to use, but it's not as bad as you think.
Put on a pair of protective eye goggles and latex gloves. Although Epoxsol is a product that has minimal odour and will not harm the respiratory system, it is corrosive to skin and eyes. So protective gear is important when using it to dissolve casting resin.
Pour a small amount of Epoxsol into a metal container. Use metal because the solvent is not corrosive to metals. Only pour what you will need to clean up the casting resin.
Apply the Epoxsol solvent directly to the resin you wish to dissolve. Let it stand according to the directions on the bottle. Generally you should wait five minutes for every 1/8 inch of thickness of casting resin.
Wipe away the stripped casting resin with a clean dry cloth. Using a spray bottle full of water, spray the area you just wiped the resin and solvent from. Thoroughly wet the area and allow the water to stand for a couple of minutes. Wipe away the water with a clean dry cloth.
Repeat the solvent process if it did not dissolve all of the casting resin the first time.
Be careful not to spill Epoxsol solvent on yourself, your clothing or other people. It is highly corrosive to skin and can cause a serious chemical burn on contact.