Regrouting works when grout is crumbling, permanently stained, or missing in sections, but it does not correct substrate movement or replace flexible movement joints. Loose material must be removed back to a sound base, dust cleaned away, and the correct grout applied with proper curing.
Distinguish grout from a flexible movement joint
Corners, tub-to-wall joints, and some changes of plane commonly need flexible sealant rather than rigid grout. Filling those areas with grout can lead to repeated cracking because the surfaces move.
Remove loose grout back to sound edges
Use an appropriate tool and control depth so waterproofing, tile edges, or hidden services are not damaged. There is no need to excavate an entire sound joint when only one section is clearly deteriorated.
Remove dust and mix according to the product
A dusty joint reduces adhesion. Clean it, leave the moisture condition required by the manufacturer, and respect the water ratio; adding extra water to make grout easier to spread can weaken it or change color.
Protect the new grout during curing
Remove haze from tile at the specified time and keep the area dry for the product's required interval. If the joint cracks again quickly, investigate movement, flexing, or moisture rather than applying another layer on top.
Sources
- American Cleaning Institute — Cleaning Tips — General household-cleaning practices and responsible product use.