The most common regrouting mistake is spreading new grout over a thin layer of old grout. The new grout has nothing solid to bond to, so it cracks and falls out within months. We remove old grout to at least two thirds of the joint depth with an oscillating grout saw.
Joints are then vacuumed, mould is treated and any loose tiles are reset before the new grout goes in. Where tiles meet the bath, tray or worktop, we finish with flexible silicone instead of grout, because those joints move.
If your grout is only dirty, grout line cleaning and restoration may be all you need. After regrouting, grout sealing protects cement grout. For showers and wet rooms, consider epoxy grout for high-moisture areas.