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Started By
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Polyurethane- Recoat within 2 hours or after 72 hours?
Posted on 8/18/22 at 10:50 am
Posted on 8/18/22 at 10:50 am
Hey all, long-time lurker, first-time poster here.
I have a question about spray polyurethane that I can't seem to find a logical answer on.
I am coating some stairs that are exposed to mild elements outside. They have already been stained so this is more of just an added layer of protection for no real reason outside of having a spray can of poly that I was itching to try.
While reading the instructions, it states that a second coat can be applied within 2 hours of the initial coat. If a second coat is not applied within 2 hours of the initial coat, a second coat can be applied after 72 hours.
Then I noticed that this is not an uncommon suggestion. I found the same line on some cans of spray paint as well as liquid stain.
Why do you need to wait 70 additional hours if you don't recoat something within 2 hours?
I assumed it was because the undercoat would need to cure but on each of the examples above, the cans state that they are fully cured in times ranging from 2 hours to 14 hours after the application.
Why add that line in the directions? Why is something recoatable for the first 2 hours but not again for 72 hours after being sprayed?
I have a question about spray polyurethane that I can't seem to find a logical answer on.
I am coating some stairs that are exposed to mild elements outside. They have already been stained so this is more of just an added layer of protection for no real reason outside of having a spray can of poly that I was itching to try.
While reading the instructions, it states that a second coat can be applied within 2 hours of the initial coat. If a second coat is not applied within 2 hours of the initial coat, a second coat can be applied after 72 hours.
Then I noticed that this is not an uncommon suggestion. I found the same line on some cans of spray paint as well as liquid stain.
Why do you need to wait 70 additional hours if you don't recoat something within 2 hours?
I assumed it was because the undercoat would need to cure but on each of the examples above, the cans state that they are fully cured in times ranging from 2 hours to 14 hours after the application.
Why add that line in the directions? Why is something recoatable for the first 2 hours but not again for 72 hours after being sprayed?
7
Posted on 8/18/22 at 10:51 am to Silver Shock
It’s whatever the can says
Usually like 8 hours
Usually like 8 hours
Posted on 8/18/22 at 10:59 am to Silver Shock
quote:
Polyurethane
The manufacturer could have a variety of reasons but in my experience you run into adhesion issues or the top coat doesn’t cure at the same rate as the under coat and you end up with “crackle” or folding. If you’re less than 2 hours the under coat is wet enough that the second coat can bind and melt into the under coat. If you’re outside of 72 it’s probably fully cured and won’t be shrinking or moving.
Posted on 8/18/22 at 11:00 am to el Gaucho
Because it is fully cured from the perspective of you touching it or using it, not from the perspective of the solvents in the poly touching it.
The redcoat window of 2 hours let’s you add more with the first coat still in a state where it can take the additional solvents of the 2nd, 3rd…. coats. Recoat after two and before 72 and you will probably end up with a cloudy finish, or solvent pop (little craters) from the first coat still offgassing.
Usually the instructions tell you to scuff after the 72 (or whatever) hours bc the additional costs will be mechanically bonded and not chemically bonded. Unlike if you recoated within the recoat window.
The redcoat window of 2 hours let’s you add more with the first coat still in a state where it can take the additional solvents of the 2nd, 3rd…. coats. Recoat after two and before 72 and you will probably end up with a cloudy finish, or solvent pop (little craters) from the first coat still offgassing.
Usually the instructions tell you to scuff after the 72 (or whatever) hours bc the additional costs will be mechanically bonded and not chemically bonded. Unlike if you recoated within the recoat window.
Posted on 8/18/22 at 11:02 am to Silver Shock
I have no real opinion to add but I've always just reapplied whenever the first coat gets tacky. Granted, it's for small home projects but I haven't had any issues.
Posted on 8/18/22 at 11:17 am to td1
quote:
Recoat after two and before 72 and you will probably end up with a cloudy finish, or solvent pop (little craters) from the first coat still offgassing
Correct answer.
Also, those dry times are established at 70 degrees.
As a general rule, every 15 degree increase in temperature cuts those dry times in half.
Therefore, that 2 hour window in 90 degree Louisiana heat is actually closer to 1 hour. The 72 hour is closer to 40 hours (temperatures drop a bit at night)
Posted on 8/18/22 at 11:38 am to Silver Shock
With solvent-based finishes there is a chemical difference between dry time and cure time. Your poly will dry in two hours, but will take up to 72 hours to cure.
Why is that? It is due to how solvent-based finishes work. Each new layer essentially "melts" the previous layer. It only kind of melts it in the sense that with the earlier layer the solvent has flashed off and left behind the finish (poly). This allows for the quick buildup of layers when applied during the drying time because you have an open window when some of the solvent is still in the earlier coat. Beyond the dry time, your poly is curing and it is much harder, chemically, for the new coat to "melt" the prior coats and create one solid finish.
Spraying another coat after the dry time and during the cure time can potentially lead to a flakey finish that doesn't adhere together properly. These are just guidelines and sometimes not following the rules can work out fine, I've had no problems with some projects. Other times it can be a big headache that makes you want to destroy whatever it is you're working on.
Finishing is easy. Finishing well is tricky.
Why is that? It is due to how solvent-based finishes work. Each new layer essentially "melts" the previous layer. It only kind of melts it in the sense that with the earlier layer the solvent has flashed off and left behind the finish (poly). This allows for the quick buildup of layers when applied during the drying time because you have an open window when some of the solvent is still in the earlier coat. Beyond the dry time, your poly is curing and it is much harder, chemically, for the new coat to "melt" the prior coats and create one solid finish.
Spraying another coat after the dry time and during the cure time can potentially lead to a flakey finish that doesn't adhere together properly. These are just guidelines and sometimes not following the rules can work out fine, I've had no problems with some projects. Other times it can be a big headache that makes you want to destroy whatever it is you're working on.
Finishing is easy. Finishing well is tricky.
This post was edited on 8/18/22 at 11:44 am
Posted on 8/18/22 at 11:38 am to Silver Shock
DP
This post was edited on 8/18/22 at 11:39 am
Posted on 8/18/22 at 12:10 pm to Silver Shock
The initial coat will eventually need to "expel gases" during the curing time. At 2 hours, the initial coat is still in a condition to merge with a second coat and it will act as a single coat for curing. After the specified time, the 2 coats will act independently. When those gases try to escape, the result will be bubbles or similar which will cause defects in the final surface.
Just follow the instructions....Or you can learn the hard way. Trust meeeeeee.
Just follow the instructions....Or you can learn the hard way. Trust meeeeeee.
Posted on 8/18/22 at 12:16 pm to Boudreaux35
I can't add anything that hasn't already been said. We have smart posters here and I'm constantly learning things.
I'm mainly just padding my post count.
I'm mainly just padding my post count.
Posted on 8/18/22 at 12:22 pm to Silver Shock
This thread is a rarity - a highly informative and factually accurate thread on the OT!
There has been a lot of useful information posted here. Thanks.
There has been a lot of useful information posted here. Thanks.
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