By The Zagosa Team. Published .
Reviewed or updated .
Granular shock can bleach acrylic, etch headrests, and wreck vinyl liners if you add it the wrong way. The good news is that the damage is nearly always avoidable. Two rules cover most of it: pre-dissolve before adding, and balance pH first.
How shock damages surfaces
Granular chlorine tipped neat onto a tub floor concentrates locally to several hundred ppm before the jets can disperse it. At that strength it bleaches almost any colour and softens vinyl.
Low-pH water makes it worse. Shocking at pH 7.0 is far harsher than the same shock at pH 7.4, because the active hypochlorous acid form is more aggressive at low pH.
The damage shows up as white patches, faded headrests, or, in vinyl liners, soft spots that grow into pinholes.
How to dose without damage
Always pre-dissolve granular shock in a bucket of tub water (about a litre per 100g of granules). Pour the dissolved solution in with the jets running.
Never scatter granules across the water surface. They sink in clumps and concentrate on whatever lies beneath.
Check pH first. Below 7.2, add alkalinity increaser and wait an hour before shocking. Above 7.8, add pH down and wait. Shock at pH 7.4 to 7.6 for the gentlest result.
Surface care after a shock
Wipe the waterline with a soft cloth weekly. Body oils and shock residues gather at the line and can stain acrylic permanently if left.
Do not use scouring pads on acrylic. Microfibre or a soft cloth only.
Replace headrests every two to three years. Even with perfect care, the constant cycling of warm water and oxidiser softens them over time. They are cheap parts, and swapping them keeps the tub looking new.
Common questions
What about non-chlorine shock?
Non-chlorine shock is gentler on surfaces, but the dissolve-first rule still stands. The granules are sharp and can scratch acrylic if they sit on it.
Is there a tub-safe alternative?
Liquid sodium hypochlorite (chlorine) and liquid hydrogen peroxide are gentler because they need no dissolving. They are less common in UK retail but worth seeking out for show tubs.