Applying new caulk over old caulk usually produces a weak repair because the new bead bonds to a dirty, slick, or partly detached surface instead of sound substrate. For a durable joint, remove failed sealant, clean, and dry before recaulk. A minor touch-up makes sense only when existing material is sound and the manufacturer allows adhesion to itself.
Do not cover caulk that is peeling or mold-damaged
If there are gaps, lifted edges, cracks, or contamination beneath the bead, a top layer only hides the failure. Remove material back to a sound surface and correct the water or movement that caused the problem.
Cured silicone is difficult to bond to, even with more silicone
Thin residue can prevent contact with tile, glass, or a countertop. Use a compatible removal method and avoid solvents that damage the substrate. Preparation usually matters more than adding thickness.
A small touch-up needs compatible product and sound material
If the manufacturer permits repair over the same sealant, clean, dry, and test adhesion in an inconspicuous area. Do not combine unknown types and assume they will bond; silicone, acrylic, and polyurethane products behave differently.
If movement opens the joint, correct the cause
A loose sink, flexing tub, or moving frame will keep breaking new beads. Repair support and fasteners, allow the area to dry, then seal with a joint designed for the expected movement.