How can I fix a large gap between wall and countertop caused by bowed walls?
Scribe it. Use a compass (or a stick with a pencil attached to it) set to the widest gap - run the compass or stick-with-pencil along the wall, marking the countertop a set distance out from the wall. Cut along the line - if done carefully, the wavy edge of the countertop should now fit the wavy wall precisely.