Replies

1. 
   Jfrostjr | Feb 19, 2006 05:00pm | #1

   My guess would be to apply a cover coat of shellac over your original work. Then go with the poly.

   I'll never forget a class with Mitch Kohanek where he said "Shellac sticks to anything. And, anything sticks to shellac."

   Jerry

   
   

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   Edited 2/19/2006 9:12 am ET by Jfrostjr

1. 
   peabo | Feb 22, 2006 03:37pm | #6

   Sorry for the delay in replying, but doesn't the shellac have to be de-waxed in order to use it with polyurethane?  Thanks.

1. 
   Jfrostjr | Feb 22, 2006 04:29pm | #7

   Yes. Sorry I didn't mention that.Jerry

2. 
   WillGeorge | Feb 26, 2006 01:04am | #8

   "Shellac sticks to anything. And, anything sticks to shellac."
   YES IF it is de-waxed!ALTHOUGH I have use a non de-waxed and the finish is still fine.. Old thing I made many years ago.. I am sure it was NOT de-waxed..Sometimes we get lucky!

   Edited 2/25/2006 5:06 pm by WillGeorge

1. 
   jimmy | Feb 26, 2006 01:11am | #9

   This will stir things up a bit.  About a year ago there was a thread on the dewaxed/wax deal with poly.  Since I only use standard varnish I decided to give poly a try over waxd shellac to see what would happen.  It worked fine on the three or four test pieces I made.  Don't know about long term, but the poly went on and dried just fine.  Mind you, I would'nt use poly over waxed shellac on a real piece, or any piece for that matter, but....comments?

1. 
   HowardAcheson | Feb 27, 2006 06:02am | #10

   Did you use the standard method for determining adhesion? 

   The industry standard is to apply the finish, let it fully cure for 3-4 weeks.  Using a sharp Exacto type knife, cut 2" long scores every 1/10th of an inch in one direction and the same 1/10th up and down (sort of like a big cross word puzzle).

   Apply a piece of scotch tape over the cross hatched area and firmly press it down.  Now slowly remove the tape.  Count the number of squares of finishing material that is removed.  Anything more than 6-8 squares removed indicates poor adhesion.  Generally a poly varnish over waxed shellac will have at least 25 or more squares removed.

   Howie.........

   Edited 2/27/2006 5:22 pm ET by HowardAcheson

1. 
   Jrad2 | Oct 02, 2018 10:13pm | #11

   Cured acrylic laquer?

   Since you seem to know a lot about te subject of adheasion have you ever tested how well polyurethane bonds to acrylic lacquer? Does fast dry poly have worse adheasion? 

2. 
   Dave45 | Feb 19, 2006 05:45pm | #2

   Were you mixing water based and oil based components?  Was the sanding sealer completely dry and clean (dust free) when you applied the poly?  I'm just throwing out some guesses here, but these have sometimes caused problems for me.

   I have about a dozen stain/finish samples in the shop right now for a job where I'm matching a new cabinet to some old work.  The samples are fine, but I made absolutely sure that each piece was completely dry and clean before I added the next coat.

   
   

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   Edited 2/19/2006 9:48 am by Dave45

3. 
   HowardAcheson | Feb 19, 2006 08:06pm | #3

   Polyurethane varnish needs 3 to 4 weeks to fully cure and develop full hardness and adhesion.  In addition, a single coat of poly varnish is not enough to develop a protective finish.  And, three coats of Tung Oil Finish does not provide a good substrate for applying a film finish. 

   Sounds like you are going down a lot of wrong roads here. 

   If you want to apply a varnish finish, you don't need to do anything more than just thin your first coat of varnish about 25% with mineral spirits.  Apply it and let it dry 48 hours.  Then lightly flat sand it with 320 paper, dust it completely and apply 2-3 more coats thinned about 10-15%.  If you then want to "rub it out", let it fully cure for 3-4 weeks. 

   Howie.........
1. 
   SteveSchoene | Feb 20, 2006 06:37am | #4

   What Howie said.  I suppose sanding sealer has some useful production purposes, but it is almost never a good thing to use in brushed on finishes.  It's not like primer for paint, it almost always weakens the final finish.  Marketing strikes again. 

1. 
   HowardAcheson | Feb 20, 2006 05:57pm | #5

   A participant on Wood Magazine's finishing forum sent a letter to Minwax asking them the benefits and cautions of use the Minwax Sanding Sealer. Also asked was whether the product contained any stearates. Here is the responce from Minwax.

        **********

   There are neither technical or practical advantages or disadvantages to the use of Minwax Sanding Sealer. It is designed for use as a sealer coat under Minwax Fast-Drying Polyurethane. Because of this, Minwax Sanding Sealer contains no stearates of any kind. It functions as a very good sealer for our polyurethane for those who like to use Sealers.

   There are no stearate-containing sealers acceptable for use under any oil-based polyurethane. Stearates will negatively affect the adhesion properties of all urethanes.

   Sanding Sealer resin must be compatible with it's top coat resin to have good adhesion. The Minwax Sealer was formulated specifically for the Fast Drying Polyurethane and may not be compatible with other brands.

   You cited that "professional opinions" that thinning polyurethane impoves adhesion. If this means "better than stearate containing sealer", then you are technically correct. Stearates will make urethane peel, and thinned polyurethane won't.

   The percent solids composition of polyurethane is regulated by the EPA persuant to Section = 183(e) of the Clean Air Act. The only comment the regulation allows is: Thinning is not recommended.

        **************

   I think their first sentence may be the most honest assessment that any company ever gave about the need or usefulness of their own product. The final sentence in the first paragraph sums it up. It satifies a marketing need for some who think that sealers have some benefit.

    Howie.........