How many coats of primer should be used on an old house?

How many coats of primer should be used on an old house?
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I forgot to state that this is the exterior of the house.
snipped-for-privacy@hotmail.com wrote:

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snipped-for-privacy@hotmail.com wrote:

scrape clean and spot prime paint free areas, then one coat of primer over everything if the underlying paint is in poor condition
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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.
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As many as it takes until you can't se through it.
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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.
snipped-for-privacy@hotmail.com wrote:

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Thanks for all of your advice. It helps.
trbo20 wrote:

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