Helpful video reference. JetReady Exterior Cleaning's video "BIOCIDE SOFT WASHING - It Gets Cleaner And Cleaner" shows the low-pressure biocide method in action.
Why The Leas is a special case
The grand Victorian and Edwardian terraces along The Leas were built for the clifftop sea view, which means their render faces the Channel head-on with no shelter. That constant salt-laden air feeds algae and black spot growth faster than render sees inland, but it also makes the render itself more porous and vulnerable to being damaged by an aggressive clean.
Soft washing, not blasting
A biocide soft wash applies a low-pressure chemical treatment that kills algae, moss and lichen spores at the root rather than just knocking the visible growth off the surface. High-pressure water on painted or pebbledash render risks forcing moisture behind the finish and stripping the paint or texture along with the dirt.
Why the visible result keeps improving after the wash
Because a biocide treatment kills the organic growth rather than just displacing it, render often keeps looking cleaner in the days after treatment as the dead spores wash away naturally in the rain, which is the effect the video above is showing.
How often it's worth doing on this stretch of coast
Given the direct sea exposure, render on The Leas and other seafront-facing terraces typically needs treating more often than render further inland, usually every 12-18 months rather than the 2-3 years that's often fine elsewhere in Kent.
Video credit: JetReady Exterior Cleaning (@jetreadyuk).