In the twenty-five plus years plus that we've been painting I don't think we've painted an exterior without pressure washing first.
On the other hand, I've spent countless (otherwise needless) hours because the painter before us failed to do so. The painter's first principle should be the same as the physician's: First do no harm. Make your customer's home easier to paint the next time around. Not harder.
Failing to properly wash an exterior before repainting is a mistake on par with an equally notorious mistake inside: namely, failing to properly prep (wash, sand, and prime) an interior oil surface before applying a water-based finish coat. In both cases the next painter up may find it necessary to do a lot of stripping. Simply put, without the proper prep the finish coats won't stick. In the case of exteriors the culprit is chalkiness. Fresh paint will not stick to a chalky surface.
Mom was right: An ounce of prevention is worth a pound of cure.
Typically, other than removing some surface grime, pressure washing should accomplish three goals: remove peeling paint; remove mildew and mold; and remove chalkiness.
We'll start with the last one first. Chalkiness is oxidized paint. The wall pictured above was extremely chalky due to a combination of factors: sun exposure and cheap paint. When I first arrived it had supposedly been washed. And It looked fine to my client. But when I ran my fingers along the stucco a thick coat of white chalk came off on my hands. I showed this to my customer. The washing company had failed to do its job. If we'd left it that way our paint job would not have lasted. We washed the entire house again, carefully calibrating the pressure. When we finished virtually all the top layer of paint had been removed. (See the photo.) The results aren't always this dramatic, of course, but the goal had been achieved: the remaining substrate was solid. Adhesion would be not be a problem. It was ready to prime and paint.
On this same house there was still a ton of mildew, typically found on the eaves and fascia (and inside too, in bathrooms, for example). Painting over mildew is an equally egregious but surprisingly common mistake because, if it's not removed, mildew and mold will grow through a new layer of paint. Then you may be stuck with it, unless you remove the entire new, compromised, layer of paint, an extraordinarily time consuming task. The ounce of prevention? Applying a solution of bleach and detergent, before applying the pressure. (This will not harm your plants, windows, etc., if the appropriate precautions are taken.)
The first goal, removing loose or peeling paint, is perhaps the most obvious. But it does take care. The trick is to gauge how aggressively to wash. Too much pressure and you chew up the underlying wood or stucco. Not enough and you leave too much work for the next step: mechanically scrapping, sanding and feathering. Then the total process becomes inefficient. Striking a balance is the key.
A couple of final notes. An experienced pressure washer diagnoses while cleaning. For example, are there dry rot or termite problems? Second, pressure washing between paint jobs, a slightly different art, is an excellent way to prolong the life of your paint job, nipping problems like mildew in the bud. Your paint will last longer, and your house will look better in the meantime.
It always surprises me, given its critical importance, when contractors ask their least experienced workers to pressure wash, when a little experience and effort can get your project off to a good, solid and efficient start.