# Problems with Polyurethane over Sanding Sealer



## Bryon (Dec 7, 2008)

I am having a problem with Minwax Polyurethane & Spar Urethane bonding properly to both Sherwin-Williams & Benjamine Moore Sanding Sealers that I have used on several, expensive hardwood doors. The problem is the Urethane-based products are flaking slightly when sanded or will peal off if tape is applied. I now know that not all sanding sealers are compatible with polyurethane finishes; from now on I will not use a sanding sealer at all, as it is a unecessary step. However, if there is any product I could apply over the polyurethane that would solve my problems without needing to sand the entire coat of poly off these doors, it would save me a great deal of time and labor. Any suggestions? 

Bryon


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## ProWallGuy (Apr 7, 2007)

Bryon said:


> if there is any product I could apply over the polyurethane that would solve my problems without needing to sand the entire coat of poly off these doors, it would save me a great deal of time and labor. Any suggestions?
> 
> Bryon


Sorry, but I don't have any real experience in sanding sealers/polys to offer you a guaranteed fix. But my first thought would be if the poly is failing due to adhesion reasons, nothing you put over it will help it to adhere. Failing is failing. Encapsulating a failing layer will only end up with 2 failing layers. I would assume you need to remove the failing layer first before applying something else. Good luck.


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## NEPS.US (Feb 6, 2008)

I always stay with the same company for poly and sealer. I would guess that the wood wasnt tacked or cleaned properly after sanding the sealer.


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## plainpainter (Nov 6, 2007)

You have to be real careful with putting urethanes over sanding sealers. Pratt&Lambert's Varmour can't go on a sanding sealer - always follow manu. advice.


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## Last Craftsman (Dec 5, 2008)

*Failing Poly...Other members what do you think?*

If you wind up having to remove the poly coat, perhaps you should try the backside of an unimportant door with some kind of stripper, you may find this a lot easier than sanding, especially if the doors have any panels or contours.

I have never stripped poly.

The peel away system may work, I have not tried it, but I have been meaning to. It comes with large sheets that you press down over the sodium hydroxide then the coating lifts of with the sheet.

Anyone else know of efficient clean stripping methods, that would make removing the coating easier than sanding the poly? And if peel away system (sodium hydroxide) will work with poly?

Also I have used DEWAXED shellac as a sealer (not a building sealer) under poly with great success on a table. Make sure you do NOT use waxed shellac, poly wont stick to it. The shellac sealed some nasties that were causing fisheye problems.

The other solution was to actually add some silcone to the poly which I thought was ironic seeing as how it was probably silicone from furniture polish that had caused the problem in the first place. The shellac kicked ass.


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## plainpainter (Nov 6, 2007)

shellac is old school for sanding sealer - although effective. I guess in the end we should all take responsibility for what we are applying. Someone that has a lot of experience wouldn't make these mistakes. If I didn't know something - I would experiment on my own stuff.


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## vermontpainter (Dec 24, 2007)

We have used Zinsser shellac based sanding sealer in combination with polys for years. Having trained a number of employees on this process, there are a couple of mistakes that can lead to disaster:

1. Sanding the sealer too fine
2. Applying the poly too heavily over the sealer

The sealer raises the grain substantially. The temptation is to sand it really fine because you can get such a hard yet soft and smooth feel on it. If you then apply the poly too liberally, it becomes an adhesion problem. 

The key is to sand the sealer only out to about 150-180 so that there is a little bit of tooth left for the poly. Otherwise its like applying poly to glass.


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## DeanV (Apr 18, 2007)

I wonder if the idea of leaving "tooth" with rougher sandpaper is accurate. One of my hobbies is building fishing rods and epoxy is used as the thread finish to hold the guides in place. One of the leading epoxy finishes was made by someone who developed epoxies for aeronautics and NASA use. He wrote an article to explain how the idea of sanding previous layers of epoxy or the rod blank to leave "tooth" actually does not do the most for adhesion. The key is a water break free surface which is best accomplished with scotch brite pads or very fine sand paper. Coarser abrasives actually do not dull as much of the surface he compared the results of using coarser abrasives to joining two pieces of metal with rivets while the waterbreak free surface was more similar to welding. The point is that the waterbreak free surface allows the epoxy to fuller wet the surface. If you prepare a rod blank with both methods, you will see the water tends to bead up and roll off the more coarsly prepared surface while on the one that is water break free, it just spreads out and wets the surface, no beading, etc. I would think the same principles apply to other finishes.


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## vermontpainter (Dec 24, 2007)

The difference would be that in the process of building up cabinet grade finishes, historically that is done by in between coat sanding, going finer in between each successive coat. With poly, it gets to a point where you can get so fine that it won't adhere to itself anymore. If you start too high on the sealer sand, you will run into this quickly. 

Epoxies used in rod construction would be a very different animal. I believe your point to probably be quite relevant to that art, but I suspect that these are apples and oranges. But out of curiosity, as finishers, water beading on a coating is usually a good thing, no?


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## DeanV (Apr 18, 2007)

You do not want water to be able to bead before you put a second coat of epoxy over a first coat (if it the first coat has had time to fully cure). If you do your second coat of epoxy the next date, the first coat is not so hard that the second coat will not adhere to it. If you wait much longer, you must scuff it and waterbreak free is best. For installing things like the handles, you want the blank to be water break free prior to putting the epoxy adhesive and handle in place.

On the finished surface, water will of course bead.


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## slickshift (Apr 8, 2007)

I'm pretty sure the BM SS is specifically *not* recommended under poly
(says right on the can...I always have to read it as I forget which can and which can't)

I know of no fix to just "add on" to the mess, just removal of the mess


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## NACE (May 16, 2008)

If the oil systems on the floor are fully cured, or at least 3 weeks old, you may consider a latex acrylic over clear over the systems, after screening. A waterbased 2 component clear for floors may work as well. It is well said here however, that you can't fix a paint (polyurethane) problem with more paint (polyurethane) Remove down to a bare surface and start over insures success.


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## Damon T (Nov 22, 2008)

sorry to hear about your problem. Polyurethanes don't adhere well to coatings with stearates. Sanding sealers typically have stearates, which act like a soap, to help with the sanding process. As another reader mentioned, waxed shellac is also a no-no under poly.
"understanding wood finishes" by Bob Flexner is an incredible book. It was recently revised, though I don't have the new copy. It goes very in depth into wood finishes, and debunks a lot of the myths and mis-information, as well as helps you understand from a chemistry perspective (in plain english) why different finishes work the way they do. I hope you don't have too many doors to sand.


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## CobraCDN (Jan 8, 2008)

If you folks want to try something different , Zinsser Quick 15. 3 coats straight. It's a quick dry and really nice to sand, almost like lacquer. Very durable, great adhesion. Nice to brush and blows with the airless rather nicely too  I've also used it to seal down silicone along counter tops before top coating with latex.

Cheers


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## Bender (Aug 10, 2008)

Damon T said:


> Polyurethanes don't adhere well to coatings with stearates. Sanding sealers typically have stearates, which act like a soap, to help with the sanding process.


Nice Damon:thumbsup:
Triple Word Score! STEARATES

If I'm not mistaken thats what causes fish eyes as well.


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## Boyfromthenorth (Jun 18, 2008)

keep in mind that there are two kinds of sanding sealer. Even Ben Moore makes two. Read the label. One is made to have a poly on top and the other is meant to function as a sealer/topcoat. I know it seems simple but check to see on the label.


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## premierpainter (Apr 17, 2007)

You don't normally need a sanding sealer with poly, by itself. If we are staining a soft wood, we will apply a sanding sealer prior to stain. Poly is straight and then sanded and re-coated.


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## daArch (Mar 15, 2008)

CobraCDN said:


> If you folks want to try something different , Zinsser Quick 15. 3 coats straight. It's a quick dry and really nice to sand, almost like lacquer. Very durable, great adhesion. Nice to brush and blows with the airless rather nicely too  I've also used it to seal down silicone along counter tops before top coating with latex.
> 
> Cheers


Back in 1990 Quick 15 was made by M H and was a real good product. Having had great experiences with it back then, I tried it after Zinsser bought it.

Sorry, but it is now a mere shadow of its former self. Once again Zinsser strikes (out)


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## Charles (Jul 1, 2008)

You shouldn't use any Polyurethanes over Sanding Sealers. It doesn't stick! If your using Sanding Sealer, you have to use a Varnish on top. If your using Polyurethanes, there self sealing so just put on a few coats. Sanding Sealer is much easier to sand and fills in the grain better compared to going straight Poly. But straight Poly is a stronger finish. Read the specs on your Poly can.


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## vermontpainter (Dec 24, 2007)

We have used this under poly for years with no problem:


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## Fry (Jan 6, 2009)

I agree with Charles.


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