Take your damaged resin mold or cast object into a well-ventilated room. Tape plastic trash bags over your work surface to protect it from the resin you will be working with.
Put on a vapor respirator, a pair of latex gloves, and safety goggles to protect yourself from the resin. Cover your arms with clothing.
Examine your damaged resin molding to determine the extent and type of damage. If the resin has been chipped or cracked, your repairs will be minimal. If any pieces have broken off completely, you may have some difficulty.
Pour a small amount of resin into a plastic cup and add the catalyst, following the instructions for your brand of resin. Stir the resin well.
Dip a paintbrush into the resin and apply the brush to the surface of your resin piece, filling in any chipped or cracked areas. Let some of the resin drizzle into deep cracks, so that the space is completely filled.
Take any pieces that have broken completely apart. Paint a small amount of resin on the broken surfaces of each piece, then fit them back together. If you still have the mold for a resin cast or the molding box for a resin mold, put them back in there so that the pieces are held together while the new resin cures.
Let the resin cure for 12 hours. Re-examine the surface of your piece to make sure that the cracks are filled and that the broken parts are sticking together again. If necessary, mix up a new batch of resin and fill the cracks again.
Replace your vapor respirator with a particle respirator, and continue wearing your safety goggles. Take a piece of sandpaper and sand down the surface of your resin object, until the new resin blends in with the old.