The Illusion of Depth in Transparent Coatings

      A simple question about easy-to-use lacquers sets off a technical discussion of the optical properties of finishes, and the characteristics that make even thin finishes appear deep and clear to the eye. May 17, 2010

I have shot many different types of finishes over the decades, but have never once shot lacquer. I now have two projects - one clear, one paint grade - and would like to try lacquer. Any nice pre-cats that are user friendly and will allow me to tint them for solid colors? Looking for the fast set and depth quality of the finish.

Forum Responses
(Finishing Forum)
From contributor D:
There are many products out there to do what you want, both in solvent base and waterbase. So let us know what you have available and what you want to spray.

From contributor A:
High quality pre-cat lacquers have been available from most suppliers since year 2000. Throwing a dart will yield success even if you are blind. That said, what's easily available? What have you being spraying besides lacquer?

From contributor L:
I find that ML Campbell's MagnaMax or MagnaKlear are very easy to work with and very forgiving. MagnaKlear is more forgiving than MM, and MM is very forgiving.

From the original questioner:
I've shot plenty of Target, and I like it but don't think I can get that lacquer look - depth, easy to repair. I'm in GA. I have a can of MagnaMax 3 years old, never opened. I read some bad things about it, so never shot it. Can I tint it white? I have some waterbased lacquer from Hood that has good reviews, but never opened it. I still think I will get that plastic look and no depth with waterbased. Very impressed with Target, but still not enough depth.

From contributor C:
What do you mean by depth? Do you mean DOI (distinction of image)? If so then any coating that has a refractive index close to the refractive index of the wood will suffice. Depth has little to do with build; it's mainly a function of refraction on the surface. Therefore a very thin coating can be applied and look miles deep. French or friction polishing is one means to this end. Nitro lacquers are a good second choice, as are the rest of the evaporative coatings.

My recommendation would be either a bar-top nitro based on non-reactive alkyd resins for gloss/depth/great adhesion, and/or a true acrylic coating based on nothing but methyl and ethyl acrylic resins with no CAB components.

From contributor C:
Look for coatings which are able to give you great depth with minimal need for build, such as shellac or nitro or acrylic polymers. Depth is acquired by how the coatings refract light as well as how they transmit the visual aspects of the substrate (wood). The higher refractive index a coating or solvent has, the more depth it will convey to the viewer. Most good coatings have refractive indexes (n/d) of 1.5 or more; for example, a normal nitro coating will be 1.51 and shellac 1.52. This will vary with the type of nitro being used and especially the solvents involved.

So when someone asks about depth, I take that to mean distinction of image - how glass-like and mirror smooth and deep the refractive and transmitted substrate appears beneath the coating.

Or, what will give me a look close to pouring a 1/4 inch of water on a piece of wood, without that kind of build?

Nothing beats friction polish for this affect, even if the surface is not perfectly flat and smooth, such as on antiques. But nitro modified non-reactive bar top nitro based lacquers come mighty close, especially when rubbed out after curing. So do unmodified straight acrylic coatings manufactured for wood use. CABs are nowhere as good as a well balanced meth and ethyl based 100% pure acrylic lacquer.

From contributor M:
I looked up the definition of "refractive," and this term refers to a substance's ability to bend light waves. So you are saying that the more a finish bends light, the thicker it looks and conveys a sense of depth? Is clarity also involved in this sensation of depth, in that some finishes may have more semi-opaque solids in them, which reflect rather than refract light, and are thus less clear?

Also, everyone uses the term "CAB lacquer." Web sources call it an acrylic product, a term which I have always associated with water based. Is it water based, and if not, what exactly does acrylic mean?

From contributor C:
Refractive means exactly what you state, but there is more going on than you expect. Distinction of image comes into play, which means how clearly you can see that which is beneath the surface. The more light is refracted and reflected, the less glossy it is, or the less intense. For instance, a flat looking finish due to additions of flatting agents make black look milky or grey in comparison to a high gloss finish that looks deep black. In a puddle of water, if the light is right in your eyes, all you see is the reflection of light. At an angle with the sun behind you, you can see all the way to the substrate beneath the water.

With flatting agents, the light becomes diffuse, meaning it refracts off the flatting particles at many different angles and makes the coating appear duller. But this has little to do with depth as we talk of DOI, in a gloss coating. Non-flatted coatings made with glossy resins and solvents with refractive indexes above 1.5 are pretty much common in wood coatings, some better than others.

The closer you match your solvents to the need for more DOI and n/d, the more clarity and depth you will have.

From the original questioner:
This is getting complicated. When I am shown a door with lacquer, it has a deep reflection like when I look through a glass bottle. On boats we used Epifanes varnish. Looks fantastic after 3 coats; however, they recommend 7-12. You would think it wouldn't get any nicer after 4 coats, but it starts taking on depth and looks better and better after each coat. Hard to explain. Clarity was achieved in 3 coats - it just becomes deeper.

I can't do this with Target, 2k polys. I can with single part varnish or poly, but can afford the dry times. So this is why I want to try lacquer. To get that 9 coat varnish effect in short dry times.

Each finish clearly has a specific advantage. WB being clarity but no amber sheen you get from oils. I thought, besides reparability, lacquer's strongest quality was depth. Not durability, not clarity. My question, I guess, was answered - just about any quality lacquer will work.

From contributor J:
If you're spraying anything that gets wet, not any quality lacquer will suffice. When I was first learning, I sprayed a re-face with piano lacquer (straight nitrocellulose). Looked great until water turned it white. Figured I'd mention this just in case you didn't know.

From contributor C:
To the original questioner: according to your last post, you are strictly talking depth relating to thickness. If that's all you're after, then yes, lacquer will give you that. Most of my pianos had 30-35 coats of lacquer on them and were thick looking. Just remember the more coats applied, the longer the through cure to rock hardness - usually 30 days or so.

CAB (cellulose acetate/butyrate) is a two component polymer resin used to increase build in straight acrylic lacquer coatings. In my opinion, they are crap! I have had many problems in the past with them, including cracking and flaking. Straight acrylic resins with proper solvents and plasticizers are my only choice. If you're not sure what you're getting, talk to the chemist to make sure the only resins in the formula are methyl and ethyl acrylates and formulated for wood use, not metal use. There may be waterbase CABs out there, but I doubt it.

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