# max time between primer and paint



## info2x (Aug 19, 2012)

What I thought was a simple question now has me all kinds of confused. I'm painting two rooms in the house and will be priming with Bulls eye 1-2-3 and painting with BM Regal Select. I've had lots of good luck with this combo in the past. 

This time the wife (and I) really doesn't know what color to pick and wants to do samples, which is fine since it's way cheaper than painting the room several times. Problem is I wanted to prime before the samples because of all the repair work these rooms have seen. I want the color to be as close to the final color as possible. Is there a max time between the primer and paint? Looking at the data sheet it shows 7 days until max hardness is this the same as max time?

Thanks!


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## Gymschu (Dec 12, 2010)

In an ideal situation you want to get your paint on at least as soon as a couple days, but, this being an interior, you're fine to wait a while. I mean, hopefully within a month you will have decisions on paint colors and can proceed?


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## info2x (Aug 19, 2012)

The idea would be to prime tomorrow and get the samples up. I'm figuring we will have our minds made up early in the week and then get the paint up no later than Saturday if I can't find the time during the week.


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## Nestor_Kelebay (Jun 17, 2008)

Info2x:

No, primer doesn't have a "best before" date by which you're supposed to paint over it.

Primer is really easy to understand. A primer's job is to stick well to the substrate and improve the adhesion of the top coat of paint. So, when they make a latex primer, they're going to choose a plastic resin that sticks well to common substrates like wood, drywall, joint compound, paint, etc. so that it sticks well to the substrate. Then they put all kinds of huge rocks in it that are almost large enough to see with the naked eye, and that makes the primer dry to a matte finish. It's entirely the increase in surface area provided by that flat finish that improves the paint's adhesion to the primer. That is, the adhesive force between the primer and the paint is the same per square inch, but with a matte finish, a primer will have more square inches of surface area than the (presumably smoother) substrate, and that's what promotes the better adhesion of the top coat of paint.

So, since that surface area doesn't change with time (or at least, it shouldn't unless someone scrapes that primer smooth with a paint scraper), paint will stick to old primer just as well as it will stick to new primer.

Where you were probably confused is where you were reading about oil based primers and oil based paints. Oil based coatings cure by reacting with oxygen in the air which causes "cross links" to form within the coating. It's that crosslinking which causes an oil based coating to transform from a liquid into a solid. If you apply your oil based paint as soon as practical over your oil based primer, then you end up with crosslinks forming BETWEEN the primer and the paint, and that greatly increases the adhesion between the two coats. You effectively have chemical bonds fusing those two coatings together; very much like the solvents which cause ABS and PVC pipes and fittings to be "cemented" together.

But, in all latex primers that I know of and all conventional latex paints, the curing is entirely due to something called "coalescence" and there's no chemical reaction involved that would result in better adhesion by painting over the primer earlier. There are lots of paints that do have a chemical cure that happens after they're dry to the touch, but I don't know of any primers that do that, so there's still no advantage in painting over latex primer with latex paint earlier than later.

Hope this helps.


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## afrig (Feb 22, 2016)

I typically don't like waiting over 48 hours but you can get away with a while probably since it is inside.


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