Rain is not clean. It picks up whatever is on the roof, the render and the frame, runs down the glass and evaporates, and everything it was carrying stays behind as a spot. On a house with a dusty pebbledash frontage, one heavy shower will undo a perfectly good clean in an afternoon. That is not a fault in the work, but it will be described as one.
Genuine hard water damage is different and worse. Where a sprinkler, a leaking gutter or an overflow has been hitting the same pane for months, the deposit is not sitting on the glass, it has begun to etch into it. You can feel it with a fingertip. No amount of pure water will move that, and it needs an acid or a cerium oxide polish, which is a separate job with a separate price.
Telling deposit from etching
The test is simple. Wet the pane and look at it side on. If the marks disappear while wet and come back as it dries, it is deposit and it will come off. If they are still visible under water, the surface itself has gone and you should say so before you start rather than after.
North Wales water is moderately hard in most of the county and noticeably harder in some of the limestone areas. It makes very little difference to pure water work, because the whole point of the resin is that you have taken the minerals out. It makes a great deal of difference to anyone still rinsing off a hose. Hardness figures for any supply area are published by the drinking water inspectorate, and they are worth knowing before you argue with a customer about it.
The conversation with the customer is the actual skill. Explain the difference between spotting and etching before you quote, point at an example on their own house, and you will not get the phone call three days later. ▪