The consequences of not sanding between coats of poly
#11
  
We're getting ready to spray a poly finish on some T&G pine boards on our walls and ceilings.  There's a ton of it.  So I'm trying to avoid sanding between coats.

I expect that the water based poly will raise the grain.  And that's fine.  It's not furniture.  I don't need a glossy smooth finish.  What's worrying me is what is the real reason for sanding between coats?

If it's just to cut down the raised grain, then I can skip it.  But if the reason is to put a texture on the layer so that you get a mechanical bond between the layers, then I can't really skip it.  If that's the case, not sanding might lead to the finish peeling off later.

So what's the real reason for sanding between coats?  And can I safely skip it?
Shut up and take your medicine, citizen.
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#12
  Re: The consequences of not sanding between coats of poly by blanning (We're getting ready ...)
As a rule, urethane resins inhibit adhesion. So the scuff sanding part is just to provide some "tooth" for the next coat to grab onto. (There's a lot of other reasons as well for the scuff sanding, but lets stick to the recoat part.) Most waterborne finishes are actually an acrylic resin finish and in some cases (my opinion) the companies put in a dollop of urethane so they can stick that seemingly magic word on the label. Collectively we woodworker's seem to regard anything labeled "poly" as the solution to, well, everything. Now to your question (you'll love this), the answer is "maybe". Many waterborne finishes advise that the subsequent coats will "burn in" to the previous coat. If you have that feature it's unlikely that scuff sanding will do anything to enhance it. But the best answer may be to follow the manufacturer's advice. Maybe even give them a call. You could do your own adhesion test, but may not want to spend the time needed to do it.
I started with absolutely nothing. Now, thanks to years of hard work, careful planning, and perseverance, I find I still have most of it left.
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#13
  Re: RE: The consequences of not sanding between coats of poly by fredhargis (As a rule, urethane ...)
(03-17-2021, 10:54 AM)fredhargis Wrote: As a rule, urethane resins inhibit adhesion. So the scuff sanding part is just to provide some "tooth" for the next coat to grab onto. (There's a lot of other reasons as well for the scuff sanding, but lets stick to the recoat part.) Most waterborne finishes are actually an acrylic resin finish and in some cases (my opinion) the companies put in a dollop of urethane so they can stick that seemingly magic word on the label. Collectively we woodworker's seem to regard anything labeled "poly" as the solution to, well, everything. Now to your question (you'll love this), the answer is "maybe". Many waterborne finishes advise that the subsequent coats will "burn in" to the previous coat. If you have that feature it's unlikely that scuff sanding will do anything to enhance it. But the best answer may be to follow the manufacturer's advice. Maybe even give them a call. You could do your own adhesion test, but may not want to spend the time needed to do it.

The instructions on most of them say not to spray them.  But people all over the internet (professionals included) claim to spray it all the time with excellent results.  The no spraying mandate is probably just lawyers skewing the instructions toward the average DIY beginner.

I could just try spraying one coat.  No sanding needed then.  But then I wonder whether I would get enough protection from that.

It's probably best to look for one that does the burn in like you said, then just skip the sanding.

Thanks for the response.
Shut up and take your medicine, citizen.
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#14
  Re: The consequences of not sanding between coats of poly by blanning (We're getting ready ...)
I would have to look at the label again but as I recall Minwax oil based says no sanding between coats if recoated within 4 hours.
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#15
  Re: RE: The consequences of not sanding between coats of poly by blanning ([quote='fredhargis' ...)
(03-17-2021, 01:01 PM)blanning Wrote: The instructions on most of them say not to spray them.  But people all over the internet (professionals included) claim to spray it all the time with excellent results.  The no spraying mandate is probably just lawyers skewing the instructions toward the average DIY beginner.

I could just try spraying one coat.  No sanding needed then.  But then I wonder whether I would get enough protection from that.

It's probably best to look for one that does the burn in like you said, then just skip the sanding.

Thanks for the response.

How much 'protection' do walls and ceilings need?

Would lacquer be a poor choice?
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#16
  Re: RE: The consequences of not sanding between coats of poly by KC ([quote='blanning' pi...)
(03-18-2021, 05:00 AM)KC Wrote: How much 'protection' do walls and ceilings need?

Would lacquer be a poor choice?

Exactly.  Fast drying and cost would be my goal for walls and ceilings.
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#17
  Re: The consequences of not sanding between coats of poly by blanning (We're getting ready ...)
The wood trim in my house has shellac finish and the finish has held up since 1953. 

Shellac smells, but the smell goes away quickly.  It dries in 20 minutes and does not raise the grain.  You can get it in various shades of clear to yellow/brown.

You can roll shellac.  You can spray shellac (they sell rattle cans of the stuff, so it can be sprayed).  No sanding between coats.  It seals knots very well.

The big complaint as a surface finish is that alcohol will damage the finish.  So don't be splashing your Dom Perignon 1988 against the walls. ($900.00 per bottle)
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#18
  Re: The consequences of not sanding between coats of poly by blanning (We're getting ready ...)
I would spray a coat of Sealcoat shellac first, then your WB topcoat.  The shellac will greatly reduce/eliminate any grain raising from the WB that follows.  Most (all?) WB finishes will bond fine without the need to sand between coats if applied within the recommended window.  

John
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#19
  Re: RE: The consequences of not sanding between coats of poly by jteneyck (I would spray a coat...)
(03-18-2021, 09:56 AM)jteneyck Wrote: I would spray a coat of Sealcoat shellac first, then your WB topcoat.  The shellac will greatly reduce/eliminate any grain raising from the WB that follows.  Most (all?) WB finishes will bond fine without the need to sand between coats if applied within the recommended window.  

John

The only down-side to SealCoat is the odor and that will dissipate in a couple of hours.  With the warmer weather he can leave the windows open and get plenty of ventilation. 

The SealCoat is water-clear.  Other shellac finishes add amber to the finish. 

From my experience horizontal surfaces (like table tops) need a very durable finish.  Vertical surfaces are much more forgiving.  I would not leave pine bare however, as it will get dirty looking over time. 

The other advantage of SealCoat compared to other shellac finishes, is that it is already diluted to the "as applied" recommendation.  Other Shellac finishes require adding solvent to the finish to reach the correct dilution.  But if an amber tint is desired, shellac is a good way to do this.  Staining pine can be a chore because of the blotching.  Amber shellac will not blotch.
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#20
  Re: RE: The consequences of not sanding between coats of poly by Cooler ([quote='jteneyck' pi...)
Sealcoat isn't exactly water clear; it adds a slightly reddish tone to most any wood.  You can make any color you want from Sealcoat shellac by adding Transtint dye to it.  The advantage is you get the color you want, it's wax free, and it's a lot cheaper than buying shellac flakes and alcohol.  You can get 2 gallons of Sealcoat shellac through HD for around $80 delivered to your door.  

FWIW, even amber and other colors of shellac will blotch on pine, maple, etc., if applied by hand.  If you spray it, however, you can avoid the problem.  



John
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