We are working on some knobs that will be fastened with a sheet metal screw driven into a hole that is part of the printed piece. Right now we are having trouble with the region around the hole being thinly filled making the screw attachment weak. We have tried several different infill patterns and densities. Is there a rule of thumb as to which pattern is best for creating a 100% infill density (or close to it)? Some patterns do not allow 100% infill density.
For starters, check which version of Slic3r you are running. If you have one of the older.910b-ish versions that shipped with many printers, try upgrading to the 1.0.0RC3 version. That fixes a specific bug relating to thin walls around round holes and thin infil sections not being fully filled.
Aside from that you could also try setting the “make complete layers ever X layers” setting more frequent.