We've got some hairline cracks forming on the walls all around the tub/shower in the bathroom - walls apparently have some sort of texturizing agent on them. Shower walls are the sectional type, & regular drywall above. Also, along the edges where the shower meets the walls, there are cracked depressions that appear to have nails (?) underneath the texturizing. What could be the cause of these fine cracks, and how do I fix them (or should I leave this to a professional)??