# Recoating Within 90 Minutes or Waiting 72 Hours



## RH (Sep 7, 2010)

Using a rattle can of water based poly to do a picture frame for Jan and noticed (once again) that it states to sand and recoat within 90 minutes, or, if unable to do so, to wait 72 hours to recoat. Brought to mind that I see that admonition from time to time but never understood the reason behind it. Does it have to do with the off gassing of the product, bonding between coats, the existing coat susceptible to damage from successive coats? 

Also, what are the ramifications of ignoring the warning and recoating in say, 24 hours? A ruined surface, a less durable one, end of life on Earth as we know it?

Maybe one of our learned paint chemists can once again chime in.


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## Jmayspaint (Mar 26, 2013)

I've often wondered about this one as well. I use rattle cans a lot for spot priming and such. Many, if not most of them have similar directions. 

The only time I've actually seen disastrous results from ignoring the recoat times was with appliance epoxy. A girl that used to work with us tried to second coat a refrigerator the next day instead of following the time restrictions. The second coat bubbled like crazy. It was a real mess. 

I have a feeling it has to do with the strong solvents in spray cans. Even WB products often contain xylene and toluene in the spray can versions.


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## straight_lines (Oct 17, 2007)

It will give you a nice crackle finish.


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## woodcoyote (Dec 30, 2012)

Water based poly? You should be fine. Just don't go too heavy on the coating. 

Let dry, scuff sand, hit it again.


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## lilpaintchic (Jul 9, 2014)

90 gets it dry enough to scuff 72 gives it time to cure out so the sanding isn't gummed up and rolling. If you don't scuff, it will not bond, it'll delaminate. I don't use much waterborne, but I've experienced it on doors....it can be a mess.


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## Ric (Oct 26, 2011)

RH said:


> Using a rattle can of water based poly to do a picture frame for Jan and noticed (once again) that it states to sand and recoat within 90 minutes, or, if unable to do so, to wait 72 hours to recoat. Brought to mind that I see that admonition from time to time but never understood the reason behind it. Does it have to do with the off gassing of the product, bonding between coats, the existing coat susceptible to damage from successive coats?
> 
> Also, what are the ramifications of ignoring the warning and recoating in say, 24 hours? A ruined surface, a less durable one, end of life on Earth as we know it?
> 
> Maybe one of our learned paint chemists can once again chime in.


Hey RH,

A similar "caveat" used to be printed on many polyurethane varnishes (brush on or aerosol) e.g. "must be recoated within 12-24 hours, if not, wait 2 weeks before recoating"...and you hit upon the reason in your questions. Polys dry to a very impervious finish - and most DIY polys are modified with other resins, such as acrylics, oils or alkyds...and these poly blends may be borne in water or solvent. Even if the main vehicle component is water, there are still "hot" solvents in the product mix (not VOC, nor solvent free) - these other resins in the mix also require solvents (other than water) to keep them liquid, and in suspension, until they need to dry hard (after application) - and JMays is right, there are medium strength solvents, (toluene and xylene), in aerosols, regardless of the primary evaporative solvent (water or solvent)...

When re-coating, a poly should either be partially cured (as in _not_"dried through"), or completely cured (as in film has reached it's maximum hardness). The reason for this is, the solvents in the second coat must be able to disrupt the final cure (of the first coat) enough to penetrate a soft film, and still provide enough "open" time to allow for the dry-through solvents to evaporate (the low-flash solvents are gonna evaporate very quickly). If applied during the period after 90 minutes, but before 72 hours, there has been too much time for the first coat to cure, the skin that has formed is more than just "soft" and the solvents in the new app won't dissolve the skin enough to allow any remaining dry-through solvents to escape - before the impervious skin begins to develop on the outermost layer of the second coat (that may be a run-on sentence, but I claim no expertise in proper punctuation...).

In this example, the dry-through solvents become, and remain, trapped beneath an ever tightening cured film (that is shrinking as it cures). The solvents that are now trapped, change with temperature and humidity. If it warms up when they are trapped, they expand beneath this shrinking "skin" and wrinkling, like corduroy, occurs. Even if the temps remain constant and wrinkling does not occur, the heavier film, with the impervious shrinking skin, will continue to hold the trapped solvents, and the underneath side of the film will remain soft and pliable until such time that the solvents can find, or force, a route of escape - and be displaced with oxygen molecules to complete the curing process...

All of this is moot after 72 hours 'cause the compositional solvents will no longer dissolve the "soft" skin, or disrupt the previous coats ability to cure. It's already cured and is now un-affected by the solvents in the package. This coating will now cure as it was intended by flashing off, in an orderly fashion, perpendicular to the applied coating.

Finally...in the first re-coat scenario (applied within 90 minutes), a "chemical" bond has been created by melding the 2 applied coats together...in the second scenario (wait 72 hours and sand), the sanding "profiles" the sealed, impervious film, and allows for a "mechanical" bond between the 2 coatings.


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