Problems With Polyurethane Over Sanding Sealer

Luke S.

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?

Thanks!

Caslon

Best thing would've been to use Minwax's own sanding sealer. Sanding sealers are important on some projects to allow even penetration of stains.

Metro M & L

Sand it off and start over. If not it will fail and youll be back.

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Ohio painter

Not sure I have much to offer as far as a solution to your situation, but I thought I would offer a word of caution.

If you have an adhesion problem between the polyurethane and the sanding sealer then applying anything over the polyurethane will not solve the adhesion problem.

I would do several tape tests and see if the polyurethane is sticking at all, if it isn't sticking then I would bite the bullet and sand the door.

As far as a solution not sure I have much to offer, I haven't run into problems using sanding sealer.

Bearded Wonder

Agreed.

CarpenterSFO

You're looking for a product that will help adhere a peeling finish coat to an undercoat. I don't know of any.

When a finish fails, your best bet is to get it off asap, before it hardens any further.

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CompleteW&D

Whenever I've stained things for my own home, I use de-waxed shellac as a sanding sealer. I generally apply two coats of a 1# cut, sanding between coats with a scotch brite type pad. Once I scotch brite it, I always wipe with a rag with a touch of colorless mineral spirits to make sure I get all the sanding dust off. I don't like tack cloths because they contain wax and can leave a wax residue that can interfere with your next step in the finishing process.

I like using the de-waxed shellac because it really makes the grain pop and it gives a nice start to the finish build. It also dries super fast and can be scuffed and coated again in about 30 minutes or less. I also like the de-waxed shellac because being alcohol based, I can use Transtint dyes to color tone my sanding sealer coats if I need to alter or augment the color a bit.

I've never had any problem with oil based products like varnish or polyurethane, or water based acrylic finishes sticking to my de-waxed shellac coats. I've never tried water based poly or lacquer.

DISCLAIMER: I'm not a professional finisher. This is just my personal experience with doing things for my own home.

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CarpenterSFO

+1 for all that. I've put every kind of clear finish over dewaxed shellac - oil-based, water-based, conversion varnish, automotive clear coats, catalyzed floor finishes. Never a failure. If you don't want to mix your own, Zinnser SealCoat is just dewaxed shellac.

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hdavis

Finishing 101 - shellac sticks to anything and anything sticks to shellac. Just don't brush lacquer over it - lacquer solvent will dissolve shellac.

And yes, shellac will stick to wax.

Sanding sealers don't have to be complicated. Chances are, you can just take whatever you're using for the finish and dilute it for a sanding sealer.

I make my own tack rags - they all leave a little residue on the surface. If you make them using the finish you're about to put on, there isn't much to worry about. These days I use microfiber towels, and skip the tack rag.

CompleteW&D

I like mixing my own. A lot of times the pre-made shellac can be old. And, in my humble experience, I find when it's super fresh, it just works better. Again, not being a professional, I have no idea why. It's just what I prefer to do.

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Nick D

READ the label on the sanding sealer. Many of them will accept alkyd varnishes, but not alkyd polyurethanes. The term referred to is "stearates" where the poly will not adhere at all.

JHC

Could be the sandpaper you are using as well.

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carzie

Sanding sealer should not be used under polyeurathane, period. If any of the previuos authors have not had a failure using non dewaxed shellac with a poly top coat count your blessings. Look at the label it will give you all kinds of usefull info. If you want a fast dry sealer coat use the Seal Coat by zinsser, it does change the color a bit to the orange side. If you're not in a hurry use the poly as a first coat but only oil based if you are going over an oil stain. If you want to go over oil stain and want use a water based poly as a finish seal the stain with the Seal Coat.

hdavis

There are all kind of good reasons to use a "sanding sealer" under polyurethane, usually revolving around tint / stain schedules, but also for "sanding", meaning any smoothing of the surface, whether sand paper, abrasive pads, steel or bronze wool, or scraper.

I don't know anyone who uses non-dewaxed shellac. I suppose if I was doing a old style shellac finish it would be OK. Other than that, I'd say 40+ years using dewaxed shellac or urethane as a sanding sealer under urethane gives me some confidence in the integrity and durability.

hdavis

Just a footnote - I don't buy anything labelled "sanding sealer".

CompleteW&D

I hate to point out the obvious.... but Zinsser's Seal Coat IS shellac.

They go on to say it's Guaranteed Compatible with ALL Finishes

http://www.rockler.com/product.cfm?page=10310

http://go.rockler.com/tech/RTD20000153AA.pdf

Hey, if Zinsser tells me it's ok to use shellac under any kind of poly, and the 80 year old cabinet maker/restorer who turned me on to shellac 40 years ago.... used it under oil based varnish, and I've never had a single failure in all that I've stained and finished using shellac as a sanding sealer, toner coat and finish build starter....

It's OK in my book.

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- John Williams

Complete Window and Door

hdavis

Bingo!