Applying Waterborne Urethane over Solventborne Stain
Solvents in an undercoat are likely to adversely affect waterborne topcoats. Shellac in between is the way to play it safe. December 6, 2011
Question
I want to seal my oil stain with quick dry oil sanding sealer, sand, and then use the Water born PolyUrethane. (I don't trust water not to lift grain or to sand as easily as the oil or to have needed build with only 2 coats on uncat wbu and recoat time on some areas.) Clearly the label says no urethane on top. They don't say why. I assume because most 2k Urethane would lift it off. I am using a waterborne urethane (non cat) that is mostly acrylic. Will I have any issues?
(Finishing Forum)
Forum Responses
From contributor B:
There are potentially all kinds of issues, beginning and ending with adhesion. Most WB's can go over oil stains if they are bone dry but they always warn to do sample and test for adhesion. From the sounds of it you probably need some sort of barrier coat over the stain though. You could use a dewaxed shellac over the stain a lot of guys use it in your situation. But still do a sample and check for any issues.
Just one rant, the grain raise issue. I've been using WB stains/dyes and WB finishes for over ten years and the grain raise issue is not, I repeat, not a big deal. Sure it sands a little harder but not much and your wood has already been sealed by your oil based stain so the grain raise shouldn’t be an issue anyway.
From contributor A:
The industry standard at the moment would be oil stain, sealcoat shellac barrier coat, and waterborne topcoat. Or you can wait three days and just use the wb topcoat.
From contributor Y:
Do you have to use an oil stain? GF has a great waterborne industrial base stain that can be matched to the color you need and recoat is around two hours. It sprays great and if you don’t wipe it, it goes on very even. I sand to 220 when I use it and get minimal grain raise. On woods that it can be a real problem like pine I wet the wood between the 180 and 220 and then let it dry. I get almost zero grain raise after that. I normally use dewaxed shellac as my seal coat in most of my applications but GF says it's not necessary for their products.
From contributor S:
Unless the manufacturer of your WB topcoat states otherwise I would strongly recommend a barrier coat of dewaxed shellac. Mix it fresh from flakes if you can. Inconsistent batches of the Zinzer seal coat have been known to cause PH problems with WB's. I also wipe surfaces finished with oil with denatured alcohol prior to proceeding with a WB topcoat. If you end up using shellac you may just want to skip your oil based sanding sealer and seal with shellac. Ideally the shellac should be as close as possible to the bottom of the schedule.
From the original questioner:
Just to see if I have this correct I am guessing shellac, as it can be applied soon after the stain. The oil sanding sealer would need four to eight hours, but then would do same thing (provided label said compatible with poly). Then it would need two hours for WB. If no sealer, I would need three days to be safe. I also noticed the Zinsser quick 15 oil sanding sealer stated that the gloss sanding sealer could do a WB top coat, but no mention about the satin version (which is all SW has in stock).
I am also having difficulty finding the shellac sealers in my area, without special ordering.
From contributor S:
The instructions provided with your oil sealer are based on the assumption that you are using a solvent based urethane. While your waterborne top coat may be marketed as a urethane or acrylic urethane the chemistry is entirely different compared to a solvent based product.
To make a long story short solvents, including mineral spirits are incompatible with WB coatings. If any residual mineral spirits remains it can and will cause problems. Play it safe. If you’re able to consult the manufacturer of the WB coating you plan to use do so. Most good manufacturers will offer advice as to known compatibility issues, and finish schedules that they have tested. I would let the oil based stain dry a minimum of 72 hours regardless of what you do.
