caulking isn’t holding up well. Are you certain it’s silicone? The most durable formulations smell strongly of vinegar when applied. Some variations on the silicone theme (called siliconized latex, or paintable silicone) don’t smell like vinegar and haven’t proven very durable in my experience. Whatever the case…

• The first step is complete removal of the old caulking.
• Next, run a couple of strips of masking tape along each side of the joint area. This way, after the caulking is blooped on and smoothed, the tape can be peeled back, leaving very even edges.
• I recommend polyurethane caulking because it combines flexibility, long working life and paintability. No other formulation does all these things so well.