Shellac Compatibility with Various Topcoats

Finishers comment on the practicality of using tinted shellac as a color coat under a clear top coat, and discuss their results when applying shellac before applying various solventborne or waterborne topcoat formulas. October 19, 2014

Question (WOODWEB Member) :
Is there any reason not to use Bins as the color coat (and primer) on cabs and then topcoat with pre-cat or poly?

Forum Responses
(Finishing Forum)
From contributor C:
Make a sample, put a piece of making tape through the middle of it, put it out in the sun for a day or two and remove the tape. You will be surprised at the color change.

From the original questioner:
Thanks. I'll give the sun test a try.

From contributor A:
The only clear coat you could apply over BIN would be waterborne. The solvents in all the other finishes will chew up the shellac. Your method used to be done with things like catalyzed primer and then topcoated with a catalyzed clear. I know a couple of guys who used Resistant primer by MLCampbell for your method. They seemed to think that the clear showed less defects than a true paint type finish. I never liked it because onsite repairs or touch ups were impossible.

From contributor R:
I did this years ago and the clients were happy with it. I have not been back past the twelve month point at which they still looked good. Pre-cat worked fine for me, but as always test your pre-cat.

From contributor N:
Although I've never coated pigmented Bin with solvent lacquer or CV, I'm pretty sure (how 's that hedging my statement) pigmented Bin is not unlike Seal Coat in that it is compatible with most top coats including ones thinned with hot thinners.

From the original questioner:
I have noticed that the Bins white is not always a consistent color though. Don't know if that is comparing old to fresh though. But then, if it changes color in the can, I imagine it would change color on a surface.

From contributor A:
I would not describe sealcoat as compatible with hot topcoats. The difference is the sealcoat is a clear and any lifting caused by the aggressive solvents may not be noticeable. Bin is obviously white and the potential lifting would cause a cloudy mess in the clear topcoat. This is all theoretical of course. We primarily use tinted clears as colorcoats.

From contributor N:
I'm spraying a house full of cabs with CV tomorrow so I'll give it a shot and see how it goes. A couple of thick coats of Bin and fairly short dry-time (45 min), top coated with a 3-4 mil coat of CV should put it to the test.

From contributor N:
Well I did as I said and two coats of Bin followed in about 45 minutes dry time by two coats of solvent CV (4) mils each coat (since I'm in the process of switching brands I was able to spray two different kinds of CV on the test board (one is thinned with lacquer thinner and one with butyl acetate)) and low and behold neither CV had any effect on the Bin. Of course all this proves is that it worked once and it doesn't come anywhere near to answering the question of future cracking from putting a harder finish on a softer one and. I sure wouldn't make a habit of it but if I was in a pinch I'd know I could use it.

From the original questioner:
I must admit my initial inquiry was in regards to water based over Bins. But, to find it holds up under hot solvents is quite interesting. It must all meld together I guess - another option.

Contributor N - thanks for taking the time and doing your test. Much appreciated. What Iím trying to figure out is aesthetically, is Bins suitable as the color coat for white cabs. It appears to stand up to hot topcoats. So, I guess that leaves us with longevity in the color? Now that I think of it, I do remember one job I did with Bins/magnamax sealer /stain/ magnamx sealer/ magnamax topcoat. It still looked good after five years. I think thatís different than just white, maybe not. I donít think top coating the Bins as soon as 45 minutes is a good idea.