Layering Polyurethane

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Forum topic by Dchip posted 08-04-2009 03:27 PM 3147 views 0 times favorited 8 replies Add to Favorites Watch
View Dchip's profile


271 posts in 3248 days

08-04-2009 03:27 PM

Topic tags/keywords: tip question finishing traditional arts and crafts

I’m in the process of hand-finishing a nightstand and just applied the first coat of polyurethane. I thinned it with a bit of mineral spirits for the first coat to enable it to flow better over the stained surface (the stain dried for about a week to ensure it wont bleed). I plan on giving the surface a light sanding before applying the second coat of full-strength poly, and repeating this procedure for a third and final coat. However, I will be away from my shop for about a week (I will have time for the second coat tonight), and would like to know what risk of flaking and chipping of the final layer of poly I run with such a gap between coats. I have read that poly coats should be applied within a certain time frame, since once the polyurethane is fully cured the adjacent coats will not bond. Would a little scuff from some 220-grit be enough to bond the two surfaces, or should I just stick with 2 layers and hope it is durable enough (the wood is poplar, so not the hardest wood there is). I was also thinking about a quick shellac coat in between the last two poly layers. Has anyone had serious problems with poly chipping and flaking? This piece will also not be in the sun. Any input is much apprecited. Thanks.

-- Dan Chiappetta, NYC,

8 replies so far

View ajosephg's profile


1880 posts in 3557 days

#1 posted 08-04-2009 03:51 PM

I’ve never had a problem with adhesion, chipping, flaking, etc. with multiple coats of poly even with several weeks between coats. I have lost track of how many coats on some projects but it is probably over 7. Normally I sand between coats, but not always. It depends on the overall thickness I want and how fast I want to achieve it. When you sand between coats you reduce the thickness of what has already been applied big time.

I definately would use shellac and poly together – two totally differrent chemical systems.

-- Joe

View Scott Bryan's profile

Scott Bryan

27250 posts in 3818 days

#2 posted 08-04-2009 04:50 PM

There is little to be gained by applying shellac after the first coat of poly. If I were going to apply shellac I would do so either as a seal coat after the stain has dried or as a finish coat. You can apply any topcoat over the shellac. You could put shellac down between the poly layers but this would simply be a waste of time and finish since the purpose of the shellac application is to either prevent the stain from bleeding into the poly topcoat or providing a surface for the poly to bond to.

Poly bonds to subsequent coats through mechanical adhesion. This is best achieved by lightly sanding with 320 grit between coats to (1) remove any dust nibs that may be in the finish; and (2) to “roughen” the previously applied layer of poly so that the layer currently being applied will adhere properly. This should be simply a very light sanding. You are not sanding with the same pressure that you would raw wood. Just enough pressure to score the surface of the poly. The time between application of subsequent layers of poly is not important, as Joe has said.

-- Challenges are what make life interesting; overcoming them is what makes life meaningful- Joshua Marine

View Derrek LeRouax's profile

Derrek LeRouax

129 posts in 3290 days

#3 posted 08-04-2009 04:52 PM

I concur. I have never had a problem with large time gaps between coats.

-- Derrek L.

View PurpLev's profile


8535 posts in 3644 days

#4 posted 08-04-2009 05:09 PM

shellac over poly has absolutely no effect, as it’s main purpose is to seal the wood (and not the poly). you should not have any issues with applying additional coats of poly at any given time after the previous coat has dried. the only time requirement between coats is that you let the previous layer dry… it can not ‘over-dry’ though…

another thing to consider is that the first couple of coats of poly are base layers that penetrate the woodgrain and also fill low spots and holes – because of that it is not recommended to sand them, but to leave them as thick as possible as “base” layers for the additional layers of poly (3rd, 4th) to even out the surface. coats 3 and 4…. should be lightly sanded to to smooth out the surface.

-- ㊍ When in doubt - There is no doubt - Go the safer route.

View Dchip's profile


271 posts in 3248 days

#5 posted 08-04-2009 05:50 PM

Thanks all for your input. This is good news, as the few projects I have finished in my short career have all been multiple layers of poly with no regard for time between layers, though I do always try to scuff the surface. None of these projects have been around long enough to observe the test of time on the poly, so I’ve turned to reading and researching to fill in the knowledge gaps, though this can often be confusing and at times overly-complicated (i.e. the shellac – though isn’t it suppose to be a universal binder, even between incompatible finishes such as oil and water-based?). I suppose there is no substitute for experience. Thanks again.

-- Dan Chiappetta, NYC,

View teenagewoodworker's profile


2727 posts in 3764 days

#6 posted 08-04-2009 07:39 PM

(i.e. the shellac – though isn’t it suppose to be a universal binder, even between incompatible finishes such as oil and water-based?)

just to answer that question. shellac is a universal bonder but remember that the different finishes have different properties. you cant put water over oil because the water based finishes are hard and oil based finishes are elasticy and the water based finish like glass put over rubber will crack. but you can put oil over water because the harder finish is underneath and supported. but then again oil needs to soak in so you would need a gel coating it your going over water based finished (with the exception of dyes) because poly wants to soak in and it cant but the gel will sit on top. shellac as a universal bonder just adheres to everything so in between different finihes they will be able to adhere to each other. but the finihses themselves still have to be compatable.

View Dchip's profile


271 posts in 3248 days

#7 posted 08-06-2009 06:15 PM

Thanks again everyone. Going back to the shellac, does anyone have a couple of examples where shellac would be useful between two coats, or is it mostly used to seal wood grain or as a finish on its own?

-- Dan Chiappetta, NYC,

View teenagewoodworker's profile


2727 posts in 3764 days

#8 posted 08-06-2009 06:27 PM

if you are using a dye and then a stain on top. especially in the case of a water based dye with a water based stain on top or an alchohol based dye with a lacquer based stain on top. ideally when you have a dye down and plan to put a stain on you should use a coat of shellac in between. the only place where that can get you in trouble is if you are using an oil based stain because the surface is sealed so the oil cant soak in. in that case a gel stain works good. also if you are using a lacquer or some other finish that is a high film finish a coat of shellac first will block any surface contaminants from ruining the finish

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