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Old 08-22-2012, 07:06 PM   #1
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Q 4 ric- old primer


Ric- this has come up several times and nobody has given a satisfactory answer.. And you're just the guy!
IT IS SAID ( sounds important..!) that a primer should be topped within the 30 day cure time.
But if it's ok to put a latex on latex without priming years after , why wouldn't it be just as fine to paint over an acrylic primer anytime?

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Old 08-22-2012, 10:52 PM   #2
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Q 4 ric- old primer


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Originally Posted by Brushjockey View Post
Ric- this has come up several times and nobody has given a satisfactory answer.. And you're just the guy!
IT IS SAID ( sounds important..!) that a primer should be topped within the 30 day cure time.
But if it's ok to put a latex on latex without priming years after , why wouldn't it be just as fine to paint over an acrylic primer anytime?
OK - to begin, we're all in agreement that primers should not be used as finishes, and finishes should not be used as primers, right?

As IT IS SAID, and for best results, primers (all types) should be top coated within X amount of days or weeks after application...The reason an oil base (alkyd) must be top coated within a specific time period has to do with the curing mechanisms of alkyds. Alkyds never really finish curing. They will continually get harder, more brittle and more impermeable (less permeable?) over time. Once the time has past that the solvents of an alkyd topcoat may no longer permeate the alkyd prime application (say 30 days or so), there will be no mechanical bond between the 2 coatings...without a mechanical bond, alkyds will ultimately lose adhesion - between coats.

So, change the scenario just a little...If you were to topcoat an aged alkyd primer with an acrylic, chances are you'd have better success since acrylics don't necessarily have to have penetration to adhere. So the chances then must be better that an acrylic will adhere to a primer that has been applied longer than X number of days or weeks...right? Um...no, not really 'cause the other really annoying attribute of alkyds are their propensity to chalk...And as good as acrylics will adhere to slick surfaces, they won't adhere to chalk - loose or semi-bound.

Of course I'm speaking very generally here and haven't differentiated between wood primers & finishes or metal primers & finishes - but all statements are primarily directed to exterior applications - these caveats may not be as much an issue for interior apps...

Now, your question has to do with an acrylic (or latex) primer and acrylic (or latex) topcoat. Acrylics cure differently than alkyds, and that cure time is pretty finite...in other words, there comes a time (usually 2 weeks to 45 days) that an acrylic finish will reach it's maximum hardness - then will harden no more. And typically, acrylics (primers or finishes) won't chalk. So, based on those truths, there really shouldn't be any reason an acrylic primer, that has been allowed to weather the elements for an entire cycle of season changes, couldn't be successfully painted with an acrylic topcoat...right? Again...um... not necessarily. Possibly...probably...it certainly would have a better chance of success than either of the other scenarios involving alkyd primers - but as I've mentioned in other posts, there are a bunches of acrylic resins available to manufacturers - and each performs differently. An acrylic primer (rust inhibiting metal primer - or - stain blocking exterior wood primer) most probably is not the same resin as is in the acrylic house paint or acrylic rust inhibitive enamel...The acrylic resins used in primers must not dry as hard as the resins in finishes...they must remain "softer" to conform to the surface being primed - these resins won't actually penetrate like an alkyd, but they will "settle" to kind of encapsulate the microscopic surface irregularities of the substrate necessary to prevent (or retard) rust (oxidation) on metal, or bind loose residual chalk or fiber and limit, or restrict, vapor transmission on wood...Because of the way all acrylics cure - once they reach the point they can do these things, that's as hard as they will get.

Since the resins in acrylic primers remain "gummy" in comparison to finishes, if left to weather through a cycle of season changes means these resins will soften in the warmer months (more so than finishes) - and thicken, or congeal, in cooler months (again, more so than finishes). As they soften, they will attract dirt, dust, pollen, bird poop, or any other of a myriad of pollutants and incorporate them into the film - as they thicken, they drive these new foreign solids deep into the film. Then the process starts all over again as temps warm up...

The way these pollutants have been "churned" into the primer film, it's near impossible to clean, thoroughly enough, to ensure that the foundation is "free of all dust, dirt, grease or foreign matter that may inhibit the adhesion of subsequently applied coatings..." or in other words, paintable.

Obviously, what I've described is an absolute worst case scenario. This most likely would not happen in 9 out 10 jobs...But in that 1 job out of 10 - whatta mess to try to clean up and redo.

I don't know if that answers your question (quite frankly, I think I've forgotten what it was anyway)...Again, keep in mind that all things mentioned tonight is scenarios and hypotheticals and worse case conditions.


Last edited by ric knows paint; 08-22-2012 at 10:55 PM.
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Old 08-23-2012, 03:06 AM   #3
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Q 4 ric- old primer


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Old 08-23-2012, 05:48 AM   #4
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Q 4 ric- old primer


Thanks ric! Uda man!
As you hinted- in interiors, (which is what i do almost exclusively) the pollutants would not be so much of an issue ( maybe in a kit.). So if i ran into a primed door for instance that had been sitting unfinished as long as it was clean it could be topped without repriming.
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Old 08-24-2012, 07:19 AM   #5
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Q 4 ric- old primer


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Originally Posted by Brushjockey View Post
Thanks ric! Uda man!
As you hinted- in interiors, (which is what i do almost exclusively) the pollutants would not be so much of an issue ( maybe in a kit.). So if i ran into a primed door for instance that had been sitting unfinished as long as it was clean it could be topped without repriming.
The chances'd be largely in your favor that there would be no problem painting over an interior acrylic primer that had been painted earlier (standard surface prep would obviously apply)...

Again, in the earlier post, I described an absolute worst case scenario in an exterior exposure...for interior, I think the most tell-tale sign that a long-ago applied primer may not be "paintable" without significant prep, would be the feel of the product...A soft, gummy feel - always dirty and never (re)hardening due to excessive heat and/or humidity, repeated cleanings with emollient containing cleaners (such as dishwashing liquids), or excessive handling (near light switches, etc.). But, as extreme as that example is, I'm pretty certain you'd visually recognize that as an area in need of special attention prior to painting...

Put another way, there is no reason, based on composition or the curing dynamic, that an application of a conventional acrylic finish paint, following standard surface prep procedures, wouldn't adhere & perform properly, over a long-ago applied acrylic primer that has been subjected to typical exposure (not those described above) and common wear & tear.

I'd say you're good to go...
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