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Top coat over varnish


Sam777

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it sort of depends on what exactly "varnish"  is. 

I consider varnish to be a category of clear film finishes. So to me, lacquer, poly and shellac are all varnishes.  I would also consider poly blends like danish oil (typically tung oil and poly thinned with mineral spirits) a varnish since in includes the film finish.

I am fairly sure that lacquer can go over most cured finishes and stains.

 

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Here is my take. Not all so called top finishes are the same and yet are used basically the same. Some products are used in various places where others can not. But poly and so called varnish are similar in characteristics but used differently. Lacquer is different than polys because with lacquers, the coats melt into each other to form one uniform coat. As with poly and varnish they lay on top of each other forming a thicker coat. Danish oils and tung oils are a totally different category because the get absorbed into the wood fibers as with top coats they lay on top of the wood fibers. Yes Danish oil has poly in it as an ingredient which makes it somewhat a top coat but is not categorized as such. Poly is more plastic and Varnish is more shellac based but with fillers.

Anyway no it is not good to apply lacquer over varnish or poly unless you sand the bottom coat and apply a dewaxed shellac to separate the 2 materials or else they will peel. It is also not good to apply poly or varnish over the top of a piece that is already top coated and cured no matter what the finish. You need to sand it to give the new top coat some tooth to adhere or it too will fail and peel. Remember wood is always moving with temps and humidity and thus makes finishing a challenge. Here is an article that can explain the differences of each finish some what. 

https://www.diynetwork.com/how-to/skills-and-know-how/painting/whats-the-difference-between-polyurethane-varnish-shellac-and-lacquer

Edited by JTTHECLOCKMAN
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That is a whole different ballgame. I explained that in my answer above. Now the answer is still NO. Again misinformation being reported. Finishing is a topic that gets thrown around so much and things are just problems waiting to happen. I am no expert so lets get that straight from the getgo. But I have done my share of woodworking and used various products for finishing. 

You say shellac. There are various types of shellac and you do not say what is being used. Is the piece that is in question something you did?? If so what type shellac did you finish with??  If it is waxed shellac which is a harder shelled finish then the answer is NO. No finish will stick to it including poly. If it is dewaxed shellac than YES you can top coat with anything. If this is flakes and you mixed with alcohol than the answer again is NO. 

If you do not know what the type finish is that you are trying to use lacquer on then resort back to my original post. Sand the project down and spray or brush on some DEWAXED shellac. This sets up a barrier between products that is safe. Or strip it down entirely and start over but I would still use a dewaxed shellac as the under base or at that time you can go with a sanding sealer. 

Now if others tell you they have used such and such and had no problems then you decide how you want to approach. 

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It is interesting to note that Zinnser makes a few, different, premixed shellac products.  They have 2 versions of their liquid shellac, Zinnser Bulls Eye Shellac (in clear and amber) and Zinnser Bull's Eye Seal Coat (clear only).  The Seal Coat product is wax free, while the plain shellac contains wax.  What is confusing, at least at first glance, is that the spray product has the same name and label design as the liquid Zinnser Bulls Eye Shellac, in the can.  This would seem to imply, again at first glance, that the spray product is the same as the liquid, thus it would contain wax.  Now, I've dug into this a little and am confident that the spray shellac is de-waxed, but I don't know why the manufacturer doesn't change the label to call it Seal Coat, so as to avoid confusion.  Or even simpler, just state it clearly on the label, either waxed or de-waxed.  But like many finish manufacturers, they seem to like to keep the consumer guessing.  🤨

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It will say somewhere on the can. Here is what Bill is talking about and the tech notes state it is not a dewaxed shellac and should not be used under poly. Look under product description. The label says sealer. I agree with Bill and if you are unaware of the differences then you can run into problems and thus I bring this up. Not all shellac is the same so when the generic word shellac is used be aware. I agree about putting the wax or dewax right on the front label. All shellac is a seal coat so that is why it is used in all their product labels. 

https://www.rockler.com/bulls-eyereg-shellac-amber

Edited by JTTHECLOCKMAN
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14 hours ago, JTTHECLOCKMAN said:

It will say right on the back 100% wax free. I believe that is what that is because it says it is a sealer too. Look at back of the can. If that be the case you are good to go. It will even say on back of can. Can be top coated. My work here is done.😀

 

zinsser-shellac-finishes-408-4f_1000.jpg

Yes, my can says exactly that. I thought I I replied to your comment last night but I guess I did not hit the send button. Thx a lot

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