The pressure washer is the instinct and it is the thing to be careful about. Black staining on siding, a fence, or a north-facing wall is organic growth with roots into a porous surface, and high pressure removes the top of it while driving the rest deeper into the grain. It looks perfect wet. It comes back darker, and the surface it comes back on is now rougher and holds more.

The distinction that matters is between grime, which sits on top, and this, which is alive and anchored. Anything anchored has to be killed and released rather than blasted, or you are just pruning it.

What a stain remover of this kind does is break the pigment and the structure chemically, so the black stops being attached and rain or a low pressure rinse takes it away. Most of the work happens while you are not touching it. That is the whole point and it is why the contractor grade version is a concentrate: the working time matters more than the scrubbing.

So the method is spray, wait, and resist. Do not scrub during the dwell. Keep the surface wet with product rather than reapplying to a dried patch, and rinse gently at the end rather than at pressure. On a hot bright day work in the shade or early, because product that flashes off dry stopped working the moment it did.

Two limits, stated plainly. This is a stain remover, not a permanent treatment. Growth returns wherever the conditions that grew it are unchanged - shade, damp, poor airflow - and this is a strong oxidiser, so it will lighten fabric awnings, kill plants at the base of the wall, and mark bare metal. Wet the plants down before you start and rinse them after. That step is not optional and it is the one everybody skips.