Just one if done right. Use a good brand, not some discount crap that will
blister in two years.
Prep is the most important part of the job. Happens to be the most boring
and tedious, but if not done properly, you'll be doing it again soon. Scrap
that loose stuff off.
A Sherwin Williams rep once told me that your primer coat should look
like a final coat. In other words, the primer should be completely
opaque with no shades of wood showing through. He also showed me a
trick where you toss a cup of water on the paint and watch what it
does. If it mostly beads up and rolls off, then your paint is in good
shape and doesn't need to be primed. If the water fairly quickly
soaks in darkening the paint, then your surface is not sealed and you
should prime the whole thing.
If you have a large area to prime (wooden shakes or lap siding), I
strongly suggest renting a sprayer. They make quick work of this kind
of thing. If the surface is rough, have someone follow your spray with
a brush. This is called backbrushing -- it helps work the primer into
the pores and crevasses.
Although I constantly hear otherwise, my opinion is that nothing beats
an oil-based primer. I generally use oil prime with a latex paint top
coat. Latex primers simply don't seem to penetrate like oil.
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