Bathroom in Harbour Blue Satin: Moisture, Mould, and Film Build

A bathroom wall fails from the inside out. Satin at LRV 26 slows vapour transmission enough to matter, but only if the film is complete — a single uncaulked 2 mm gap at the bath edge will put more moisture into the wall than the paint ever keeps out.

Coats

2

Sheen

Satin · 34u

Substrate

Old paint

Touch dry

2 hours

Recoat after

5 hours

VOC

44 g/L

Full cure

14 days

What actually kills bathroom paint

It is rarely the steam in the air. Airborne moisture leaves with the extractor. The damage comes from water that gets behind the film and cannot get out again: an unsealed joint at the bath edge, a crack at the tile line, a screw fixing that was never filled. Once water is behind a paint film, the film blisters from the substrate up, and no amount of surface preparation prevents it.

That is why the order of work in a bathroom is: seal the edges first, then paint the field. Caulking after painting means the caulk line sits on top of the paint, and within a year it will pull away and take the paint edge with it.

Why satin, and why LRV 26

Sheen	Water vapour resistance	Wipe / scrub	Typical bathroom use
Matte	Lowest	Poor, burnishes	Dry zones only
Eggshell	Medium	Good	Bathrooms with a working extractor and a window
Satin	Highest of the three water-based options	Very good	Wet zones, small bathrooms, no window

Satin also holds up to the mildly alkaline cleaners people use on bathroom walls, which matte does not: alkaline cleaners soften a matte film and leave streaks within a few months.

Harbour Blue (#7C8F99, LRV 26) is a grey-blue chosen for two practical reasons. At LRV 26 it does not show the fine soap film that accumulates on pale bathroom walls. And its grey cast means it stays cool-toned under the yellowish light that most bathroom fittings produce, instead of turning green the way a pure blue does.

Prep sequence

1. Treat black spotting with an anti-mould surface cleaner. Leave it the full dwell time, wipe off, and let the surface dry 24 hours. Painting over active mould is a guarantee of a recurrence within one heating season — the paint does not kill it, it just hides it.
2. Scrape and sand any blistering paint back to sound film. Feather the edges, then sand the whole wall with P180. Satin needs a mechanical key.
3. Fill any fixing holes, sand flush, spot-prime.
4. Caulk the wall-to-tile joint and the wall-to-bath joint with a fungicidal acrylic. Smooth with a wet finger and let it cure 2 hours. Do not use silicone: paint will not adhere over it and the joint becomes a visible grey line.
5. Mask the tile line and the bath rim. Run the extractor for the entire painting and cure period.

Coats and the drying arithmetic

Two coats, five hours apart at 20 C. Bathrooms are almost always cooler and damper than the rest of the house, so the re-coat window is longer than the tin suggests:

• 20 C / 50% RH: touch dry 2 hours, recoat 5 hours, wipe-clean at 7 days, cure 14 days.
• 15 C / 70% RH: touch dry 3 hours, recoat 9 hours, wipe-clean at 12 days, cure 21 days.

If the room is below 15 C, do not paint. Satin that cures cold forms a softer film and will fail at the bath edge within a year, in exactly the place where a failure is most expensive to fix.

A 2.4 x 2 m bathroom, four walls to a height of 2.4 m, two coats, with the wall above the bath in the same colour: about 5 litres of satin and one tube of caulk. Walls above 2.1 m in a bathroom are wasted paint in most cases — most of the visible wall is below eye level and the top strip is the part that gets least cleaning.

What matters here

• Caulk the bath and tile joints before painting, not after.
• Treat mould and let it dry 24 hours — paint over active mould always returns.
• Satin resists alkaline bathroom cleaners; matte softens and streaks.
• Below 15 C, do not paint a bathroom — cold-cured satin fails at the bath edge.

Elsewhere on this wall

• Harbour Blue in the palette
• Cool blues and greys