Re-caulking a Japanese Unit Bath: Sealant Choice and Cure Time

A tenant in one of my units kept apologising for the black mould in the corner of her bathroom, convinced she was not cleaning hard enough. She was cleaning fine. The silicone bead where the wall panels meet the tub had failed, water was wicking behind it, and the mould was growing in a place no sponge could reach. I stripped and re-ran that joint in about 40 minutes, using roughly 280 yen of sealant, and the corner stayed clean for the rest of her tenancy. Across 9 properties with unit baths, I re-caulk on a rotation whether a tenant complains or not.

Why unit-bath joints fail on a cycle

A Japanese unit bath, the moulded unit basu, flexes a little every time it fills and drains, and the panels expand and contract with the temperature swing between a hot shower and a cold morning. The factory silicone holds that movement for about 8 to 12 years, then it loses elasticity, pulls away from one face, and opens a hairline gap. Once water gets behind the bead, mould grows in the gap and no surface cleaner will ever reach it. That is the point to replace the bead, not to scrub harder. You can usually confirm it by pressing the joint with a fingertip: a sound bead is firm and stays put, a failed one is spongy, lifts at one edge, or shows a dark line running along the back of the gap. A tenant who keeps apologising for mould in the same corner is almost always describing a failed bead, not a cleaning habit.

Stripping the old bead completely

This is the step people skip, and skipping it is why a re-caulk fails in a year. New silicone does not bond to old silicone. I cut both edges of the old bead with a fresh utility blade, peel it out in one strip where I can, then scrape the residue with a plastic razor so I do not scratch the acrylic panel. A dedicated silicone-remover gel, about 800 yen a tube, softens stubborn film in 30 minutes and saves a lot of scraping on a joint that has been recaulked twice before. Any specks left behind get rubbed off with a cloth dampened in isopropyl alcohol. Then the joint has to dry completely, which in a windowless bathroom means running the vent fan for at least 3 hours or leaving it overnight. If there is black mould in the substrate behind where the old bead was, I wipe it with a diluted bleach solution and let that dry too, because sealing mould in under fresh silicone just lets it grow through.

Choosing the sealant

For the wet joints in a bathroom I use a mould-resistant silicone, labelled bou-kabi, in translucent or white. Not acrylic latex caulk, which is fine for a painted skirting board but will not survive standing water. Not the cheap general-purpose silicone either, because it lacks the fungicide and goes black in a season. One 330ml cartridge costs me about 700 yen and does four to five joint runs in a typical unit bath, so the material cost per joint is around 280 yen. For any gap wider than about 5mm I push in a foam backer rod first, because a deep slug of silicone never cures properly in the middle and slumps out of the joint.

Masking, tooling, and the 24-hour rule

The finish is all in the prep and the timing:

• Mask both sides of the joint with tape, leaving a gap the width you want the finished bead
• Lay a continuous bead, no stops and starts, holding the gun at 45 degrees
• Tool it once with a wet fingertip or a rubber spatula, in one smooth pass
• Pull the masking tape immediately, while the silicone is still wet, then leave it alone
• No water on the joint for a full 24 hours, so I schedule this between tenants or tell the tenant not to use the shower that night

The joints to leave alone

Not every line of sealant in a unit bath is yours to redo. The joint between the bath pan and the floor of the room outside, and any seam that is actually a structural panel joint holding the unit together, should be left to a bathroom contractor, because if you open the wrong one you can let water into the cavity behind the moulded shell where you will never get it dry. I stick to the visible internal joints: tub-to-wall skirt, the vertical corners up to about shoulder height, around the tap and shower-rail escutcheons, and the front edge of the bath apron. Anything below the floor grate or behind an access panel I photograph and hand to a pro.

Cost and time per unit

A full unit bath has maybe five joint runs worth replacing: tub-to-wall, wall-to-wall corners, and the floor pan edge. Doing all of them takes me about 2 hours plus the cure time, and costs under 1,500 yen in materials once you count the tube, the masking tape, and a fresh blade. A contractor quoted one of my properties 18,000 yen for the same job. I now re-caulk every unit bath on a 6 to 8 year cycle during a turnover, before the bead actually fails, which keeps mould complaints and water damage off my list entirely. I also keep a photo log of when each of my bathrooms was last done, so I am working from a date and not from memory when I plan a turnover.

It is a small, unglamorous job, but a failed bath joint is how slow water damage starts, so it stays on my turnover checklist for every property.

同じカテゴリの記事を読む

8棟自主管理の経験から書いた関連記事 — まとめて読むと理解が深まります

▸ DIY Repairs

Diagnosing a Squeaky Japanese Floor by Joist Deflection

▸ English

Deck Sun Protection: Shade Sails and Lattice Screens

▸ English

How to Reduce Vacancy Periods Between Tenants

▸ English

Installing an IH Cooktop in a Rental Kitchen: Amperage and Cutout

▸ Bathroom DIY

Bathroom Renovation Vol.13: Under-Floor Storage Pt.2

▸ English

Wood Deck Build #34: Steel Pipe Painting and Priming u2014 DIY Dad