The crackle medium may pull off when you coat it with a second color of paint or a polyurethane sealer if it is not completely dry. Follow product instructions for best results, and don't go over the crackle layer more than once with the paintbrush or roller.
An uneven layer of crackling medium may cause the size of the cracks in your crackled paint to vary, especially if you are crackling a large area. Apply an even layer of crackling medium with a paintbrush or roller. Don't reapply the medium over areas that you've already coated.
If the base coat begins to peel when you apply the crackle paint, all the paint layers will eventually flake away. Clean the surface completely before applying the base coat. If the surface is already painted, stained or varnished, apply a deglosser if necessary and sand thoroughly before painting.
If the base coat becomes gummy or develops small globules rather than cracks after you apply the crackling medium, it may not have been completely dry, or you may be using the wrong kind of paint. Always check the instructions on the crackling medium to see if it works with water-based paint, oil-based paint or both.