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I'm about to polyurethane my floors, and all the instructions I've come across (including the ones on the can) say to "maintain a wet edge".

What is a wet edge and how can I maintain it?

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2 Answers 2

up vote 7 down vote accepted

To elaborate a little more. When you apply a finish like poly or even paint, it doesn't just instantly go from a liquid to a solid after an hour or whatever the dry time is for the product.

When it's exposed to air it slowly starts to solidify. As this happens the physical properties of the finish change. Most importantly it's workability changes. If you brush on a section of poly and it goes on smoothly it will eventually level itself out so that there are no brush marks.

In the middle of your project the phone rings and you step away for 10 minutes. In that time the finish has begun to set up. If you try and continue the job by brushing where you left off as your brush goes over the partially dried finish it can gouge it leaving brush marks or possibly lift it off completely in some spots.

Most finishes don't like to be over worked because doing so compromises their ability to level and possibly their adhesion. Your brush should only go over an area that was recently brushed or allowed to fully dry if you can't finish what you started.

To paraphrase what Les said and Chris Cudmore said Wet on wet > Wet on Dry > Wet on partially dry. You can minimize the effect of banding when doing wet over dry by feathering the new finish over the old a bit.

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Thanks, I think your addition of wet on dry being better than wet on partially dry is a key distinction that makes this a better answer. Any idea how long I have before polyurethane starts to set up? –  virtualxtc Mar 4 at 8:01

It simply means that the edge of the finished area should not be allowed to dry out, so you're not putting wet polyurethane over dry.

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I thought it was so you're not putting poly (or paint, etc.) over PARTIALLY dry poly, thus causing the partially dry material to peel up or otherwise disturb the finish. Wet on wet = good. Wet on dry = good. Wet on partially dry = bad. –  Les Mar 3 at 14:10
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Wet over dry will usually leave a band where the overlap is. –  Chris Cudmore Mar 3 at 14:21
    
Chris is right, wet over dry is not preferred on floor areas. the "lap" created by going over the dry will show itself on large flat areas. On smaller areas (not floors) you can get away with it a little easier, because you can at least get the wet finish to a corner or some other place that will hide the lap –  Jack Mar 3 at 15:06
    
Ok, I'll modify my comment: Wet on dry = potential bad. It may leave a stripe where you overlap the finish. –  Les Mar 5 at 4:51
    
@Les Fair enough. Issue is cosmetic. –  Chris Cudmore Mar 5 at 13:57

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