# Outdoor primer for indoors



## snc921 (Jan 12, 2009)

I have some Behr outdoor primer left over from painting my doors.

Is it ok for indoor use, on patched plaster? Or should I buy new indoor primer?

My guess is it's ok, but I don't want to ruin the walls or have it look grossly different from the rest of the room.


Thanks.


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## Nestor_Kelebay (Jun 17, 2008)

Is it a latex primer or an oil based primer.

There really is no significant difference between indoor and outdoor latex primers. Typically, if you see a primer being marketed as an "indoor latex primer" or an "outdoor latex primer", the difference is whether it's low odor or not. They'll market low odor latex primers as "indoor primers" cuz some people don't like the smell of Texanol (the most commonly used coalescing solvent). And, of course, non-low-odor primers are "outdoor" primers. I'd use any latex primer indoors or out. You'll also find that most latex primers advertise themselves as "indoor/outdoor" primers, cuz very few people are bothered by the smell of latex primers or paints.

Oil based primers are a different thing. Exterior oil based primers, just like exterior oil based paints, will dry to a softer film that can stretch and shrink with wood outdoors. Wood is a natural material and it's moisture content changes with seasonal changes in humidity. As the moisture content in wood increases, the cells walls get softer and swell up. That results in dimensional changes in the wood that interior oil based primers and paints aren't elastic enough to accomodate. The result is that interior oil based paints and primers will crack and peel over WOOD outdoors.

So, exterior oil based primers are made especially to be softer and more elastic so that they can accomodate dimensional changes in the substrate without cracking and peeling. So, you can use an INTERIOR oil based primer outdoors if you're painting over concrete or metal or fiberglass or anything else that doesn't shrink or swell (with temperature or humidity) nearly as much as wood does.

NEITHER indoor nor outdoor primers will have much in the way of biocides or UV blockers in them because both are meant to be covered with paint, and exterior latex paints, just like exterior oil based paints, are different from their interior counterparts in that they will have much more biocides and UV blockers in them.

You can use an exterior oil based primer over drywall or joint compound inside. Ditto if you're not priming a working surface, like a wood table, shelf, window sill, mantle, etc. Where you're going to be top coating with a paint that will dry to a strong hard film like you need on a working surface, you should be using a primer that will dry strong and hard too so that it will provide proper support to the paint film.

Hope this helps.


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## slickshift (Aug 25, 2005)

Do not use any paint or primer labeled for exterior on an inside project
(aside from some int/ext coatings..and they will be labeled as such)


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