Waterborne Lacquer over Solvent Lacquer

      Product reps won't promise good performance, but some finishers have had good results by cleaning and sanding for a strong mechanical bond. September 3, 2011

Question
I had a long discussion with the MLC rep today about putting waterborne lacquers over solvent lacquers but I wanted to get some of your opinions on the matter as well. I primarily do refinishing when it comes to woodwork, and most of that is on-site work in occupied homes. Therefore I would much rather entirely switch over to WB systems for several reasons not the least of which are potential fire hazards and off-gassing issues.

What are your thoughts on putting WB coatings over SB coatings? Is there a way to make it work? MLC seems to think that a WB coating over an existing SB finish would last a year at best before it started flaking off, which is entirely unacceptable in my opinion. Have any of you found a good system for going over existing finishes with WB lacquers?

Forum Responses
(Finishing Forum)
Mike Schwartz
From contributor Z:
With waterborne coatings it is always best to take the advice of the manufacture in regards to compatibility with a given solvent system. If you don't trust their advice, or they won't give an opinion I would prepare a few samples, and pre-form an adhesion test, and test for moisture/chemical resistance after allowing them to fully cure.



From contributor L:
Mechanical adhesion is what you need. If the two are uncatalyzed materials, then I think you should be good to go. I'd test and re-test your system(s) to be sure though. You never know what will become of your work once it leaves your shop.


From contributor F:
If the old lacquer is in good shape, give it a good cleaning followed by a 320 scuff and then a coat of solvent based vinyl sealer. Then scuff the sealer before top coating with WB.


From contributor N:
He is covering his butt in case you use his products and have an issue. I specialize in this sort of thing and have sprayed WB over Varnish, Poly, lacquer etc and if you do the prep work right it will work. You obviously want to do an adhesion test which is best done on the back side of a door but in 12 years I know even the early kitchens I did are still holding up well. The big thing is to find the right products because WB coatings are available in a wide array of different resins and formulations.


From the original questioner:
He is covering his butt in case you use his products and have an issue. That's kind of what I thought, but I even mentioned that when I was talking to the rep and basically just said "well what if I clean the living snot out of the surface and give it a very thorough sanding?" - his response was the same "I can't guarantee..."

What are your thoughts on the Agualente/Aguabarnice primers and topcoat systems? Good enough for topcoating over SB finishes? I would really like to find a once-and-for-all solution to this since I am always so worried about using solvent borne materials in occupied homes.



From contributor H:
One of my key go to rules over the years has been that "shellac sticks to anything and anything sticks to shellac". Now I have never applied that theory to lacquer, either solvent or water based. So for the first time have to wonder if it will hold true. It might be worth a try though to put a seal coat of shellac over the solvent based lacquer and see how well it does. If it bonds then I think it is even more likely the water base lacquer will adhere to the shellac.


From contributor N:
I agree with and support your statement. The only issue that I see is that Shellac is not a very good product to spray in an existing home as the OP indicated he did. In the end I would have written into your contract whatever expectations you want the customer to have. If you feel you want to cover your butt you might put in a disclaimer that "due to potential unknown contamination and composition of the existing finish we will do everything we can to ensure the process is durable and long lasting, but we are not responsible for issues that are beyond our control."


From contributor H:
You're right. I re-read the original post and shellac really isn't any improvement over the solvent lacquer in terms of solving his problem of finishing in an existing living space. I guess he could brush it on but I suspect that wouldn't be worth the effort or give as good a surface.



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