Caulk that fails is usually a dirty joint
I ran a bead along the bath in February, wiped it with a wet finger, and called it done. A month later the bead had let go at the back corner. A grey lip, a dark line underneath, and water sitting where I thought I had sealed. I pulled the new caulk off in one string. The tile still had last year's soap film on it. I had sealed dirt.
Caulk is a flexible bead for a joint that moves. A bath flexes when you sit in it. A window frame moves with heat and a shut sash. The bead has to stick to both sides and stretch a little. It will not stick to soap, mould, dust, or the skin of an old bead you have not fully taken off. People blame the tube. The tube was doing a seal's job on a greasy line.
Filler that shrinks is usually the deep hole is a different paste. Filler goes off hard in a hole. It is not for a joint that opens and closes. I have used filler along a bath because it was what I had. It cracked in a week. Caulk in a deep wall hole slumps and never sands. Use the one that matches the job.
When I redo a joint I take the old bead out. All of it. A plastic scraper, patience, and then a wipe with something that cuts the soap, not just a damp cloth that moves the scum along. The surfaces need to be dry. A wet tile looks clean and then the new bead sits on a film of water. I wait. In a bathroom that means a window open and the bath unused for a bit, not a hairdryer waved at a puddle.
Tape helps me keep the bead where I meant it. A strip on the tile, a strip on the bath, a gap the width of the joint. Gun the bead, tool it, pull the tape while the caulk is still soft. A ragged edge is a place water gets under. I have skipped the tape because I was late. I spent longer cleaning the smear off the enamel than I would have spent taping.
I also look at the gap. A hairline wants a thin bead. A hole you can lose a pencil in wants a backing, or two passes, not one fat sausage that skins and stays wet in the middle. That sausage peels the same way a dirty joint peels. The face looks finished. The sides never held.
Skip caulking over black mould and hoping the new white hides it. The mould is still there and the bead lets go. Skip a wet finger as the only tool if you have just washed up in the same water. You have put grease back on. Skip "kitchen and bathroom" on a tube if you have not read whether it is for wet rooms or only for a windowsill. Skip painting over a bead that is still tacky because you want the room back tonight. It will wrinkle and then it will peel.
UK year matters for drying, not for a catalogue. Winter bathrooms stay wet. Condensation on a window in January will sit in a joint you sealed at tea time. Summer with the windows open dries the face faster. That is when I rush and skip the clean. I now do the joint on a day the room can stay unused, not on a Sunday night before work.
I do not need a different colour because the last bead peeled. I need both faces clean, dry, and free of the last attempt.
If I later add a buy link here, it will be a paid promo.