= 20 yr old exterior paint- spot prime or prime whole house? =

Talked to sherwin williams. They recommended priming only the bare wood areas where we sanded with oil based primer, then 2 coats of paint. The first coat will "act like a primer"

I'm wondering about the remaining old paint. If that paint is 20 yr old, is it still useful for the new paint to bond to, or is it better to just prime the whole thing?

If the old paint sticks, it'll hold for a while. Putting a coat of primer over it before the new paint won't really extend its lifetime

If paint is cracking, peeling, bubbling etc, you'll need to remove/sand/treat that area, spot prime n paint 2 coats

If the whole wall is like that, you might even need a Painteater, heat gun, etc to treat the entire wall, then full prime n paint 2 coats

If the paint is solid, stable, wash, dry fully. You can usually get away with paint 2 coats, but there's never any question that a primed surface will absolutely hold paint. The reason is while most old paint in good condition will hold new paint fine, you never really know if that one so-so area that looks fine to the eye is on the verge of failing

Surface prep also greatly affects adhesion.and don't wash well, paint can peel the same year

Areas that get less direct sun and water exposure can get away with no primer, even 1 coat. Some ares can even get away with no washing

Areas that get direct sun and tons of rain - prep well, prime, 2 coats

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