Soft wash mix ratios, explained properly

Half the "my mix isn't working" problems in this trade are arithmetic. The other half are old chemical.

## Stock versus working strength Australian pool-supply sodium hypochlorite is usually 12.5%. You never apply it at that.

Above about 6% you are damaging paint, plants and yourself with no extra cleaning benefit. Strength is not the answer. Dwell time is.

## The formula Litres of stock = (target % ÷ stock %) × total litres.

For 100 L at 3% from 12.5% stock: (3 ÷ 12.5) × 100 = 24 L of SH and 76 L of water.

## Surfactant 10 to 20 mL per litre of finished mix. Without it the solution sheets straight off a vertical wall and you get vertical stripes of clean between dirty ones. With it, the mix clings and dwells where you put it.

## The downstream injector trap This is the one that confuses everybody.

A downstream injector dilutes your bucket mix again, typically at 10:1 to 20:1. So if your bucket is at 3% and the injector draws at 15:1, the wall is receiving about 0.2% — nowhere near enough. You conclude the chemical is weak. It is not; the maths is.

Test your injector once. Fill a bucket, mark the level, run for a measured 60 seconds through the low-pressure tip, and see how much was drawn. Now you know your real ratio and can mix backwards from the target.

With 12.5% stock and a 15:1 injector, you would run near-neat stock in the bucket to land around 1% on the wall. That is normal, and it is precisely why 12 volt soft wash systems exist.

## Storage Sodium hypochlorite degrades with heat, light and time. A drum sitting in a hot ute for a month will test noticeably weak. Buy fresh, buy often, store it in the shade.

## The rules that keep you out of hospital Never mix sodium hypochlorite with any acid or ammonia. That produces chlorine gas.

Wet the plants before, protect during, rinse after. Full PPE. Contain the runoff and keep it out of the stormwater drain.