Barrier Coat Between Aniline Dye and Waterborne Urethane Topcoat
Finishers give advice on how to prevent a brushed-on waterborne urethane topcoat from re-dissolving and streaking a dye coat. September 29, 2009
Question
I operate a small custom shop and usually send all my finishing out to a local guy who I trust. I'm working on one job where we have gotten into aniline dyes. I'm going to need to do some on-site finishing once everything has been trimmed out, so I planned on using a waterbased urethane. There is one I use quite a bit in restaurant jobs that dries quickly with near zero odor.
When I apply the first coat of urethane over the dye, the color streaks. The client requests that the coat is brushed, not sprayed. I assume that I am re-wetting the dye and this is causing the streaking. The dye was left to dry for around 28 hours.
Is there a way that I can use aniline dyes and waterbased urethane, and avoid this streaking? Is it a matter of spraying the first coat? I haven't tried a shellac sealer coat, but would be interested to know if anyone thinks this is the answer. My finisher isn't any help. He's good, but his world is lacquer, lacquer, lacquer.
Forum Responses
(Finishing Forum)
From contributor A:
Dye stain comes in different types. I am guessing that you used a waterbased dye? If so, the dye is re-wetting with the topcoat. You can use a shellac barrier coat (probably your best bet in the field, you can even get aerosol cans of shellac) or switch to an NGR or alcohol based dye stain which will not re-wet as easily with water.
From contributor B:
Use shellac, dewaxed. SealCoat works very well. I'd wait 12 hours for the stain to dry (although that is probably overkill) and overnight or 12 hours for the SealCoat to dry. However the SealCoat will probably draw some stain out also unless you spray it. Try a good sized sample first to be sure.
From contributor C:
If you are using water soluble dye, the alcohol in the shellac will lift it somewhat so don't overbrush the shellac. Some folks add a waterbase binder (your urethane should work) to the dye solution - 25% or so by volume - to lock down the dye from the shellac. It acts as a diluent to the dye so you'll have to jigger the mix to get your dye intensity right.
From the original questioner:
Thanks everyone. Yes, it is a waterbased dye. It sounds like a shellac seal coat is the way to go.
To contributor C: Can you clarify "add 25% WB urethane to the dye mix"? If I do this, couldn't I just go to the waterbase urethane finishing coats, without the shellac seal coat? If this was the case, I'm sure you would have said so, but would the 25% urethane not 'lock down' the color enough?
From contributor C:
You could try a test going over the 'polymeric' dye with the urethane. Some of them have a degree of 'burn' or chemical adhesion that could melt the binder content and lift the dye which is why I didn't mention it. I've sprayed tons of waterbase over waterbase polymeric dyes with no lifting problem except with the '100% burn-in types' (NMP listed on the MSDS).
The mixture based on a quart of stain at a typical dye dilution rate of 1 oz dye to 1 quart water would be:
1 oz by weight dye powder
24 liquid oz of water
32 divided by four or 8 oz waterbase sealer liquid
8 oz times the solids content of waterbase sealer (say 33%) or 2.5 oz add'l water to get full diluent factor.
