Do Not Power wash Cedar siding or any made out of wood.
I can assume there is instant dissent with the title of this story. But there is good reason not to powerwash a wood-sided exterior.
(1st) Cedar is a relatively soft wood. Whether it is White or Red Cedar or Clapboard or even Alaskan Yellow cedar, it is still wood, has longitudinal pores running its length from Butt to feathered edge on a shingle as well as on cedar clapboard.
(2) Spraying with a power washer is akin to taking a chisel and chipping, gouging into the cedar. Yes water under pressure and shot out of a nozzle at high velocity will cut thru just about anything given a long enough time span. Much faster with a wood product.
(3) That powerwash driven spray or jet of water, will get driven thru any existing cracks, fissures or just nail holes, not to mention gaps between clapboard courses as well as shingles. That will result in excessive moisture residing in between the (back or surface facing the sheathing wall) shingles or clapboard and the housewrap.
This moisture will eventually seep out from undercover, but it will do so long after the siding has appeared to be dry enough to paint.
The result will be spots, patches, area’s on the field (wall) which will have recently applied paint peeling or bubbling.
So the answer is to prep an exterior wall for coats of new paint the old fashioned method, scrape, sand, fill in gaps with latex/silicone exterior caulk, the type that contracts and expands with temperature variations, yet still adheres strongly to the joint or crack that it was initially applied to.
Then smooth out (first when wet during application), a damp sponge works best for feathering the edges of the caulk and smoothing it over the imperfection.
However if the Clapboard has a crack that has separated from the board then it is to be replaced, perhaps not entirely but at least to the next stud (nailer) after the crack ends.
Shingles, if they are split (cross-split), have missing broken off corners should be removed with a shingle thief and replaced with a fresh shingle.
How to nail a shingle in an existing wall course….?? Here is a trick we use.
Once the bad shingle is removed and also it’s nails which should be on average hidden 5 to 5-1/2 inches up on the next shingle course you can slip the new cut to size shingle in place, tap it up but only until the butt of the replacement shingle is about 3/8ths below the corresponding course. Now nail up two shingle nails, 1/2 inch in from the side edge of the shingle directly below the butt of the next shingle course.
BUT this is the important part, nail the nails at a 45 degree angle, in other words the nail head points downward so that the nail shaft is at a 45 degree angle to the vertical wall. Nail all the way flush with the shingle, the top of the nail head should be just below, actually touching the above shingle butt of the next course. You will need a nail set to drive the nail in flush so as to not damage the surrounding shingle’s. Once the nails are flush, tap the bottom of the replacement shingle upward, thereby driving the shingle up flush with its course line. As you do this, the shingle moving up will also drive the nails up and under the above course. That’s it new shingle on with no visible nail heads.