Differing Finish Layers

[Sal Giambruno]

Are there any general guidelines when it comes to using different finishes atop one another? I remember reading somewhere that putting a harder finish on a softer undercoat is not good, but that's about all I know. What's more, I wouldn't know a harder finish from a softer finish due to lack of experience.

I'm thinking of maybe spraying a coat or two of solvent lacquer (or something else) on top of a couple of existing coats of Varnish and am wondering if that's a risky proposition. My goal is to achieve a rich, deep, smooth, glossy finish while having to do as little (or no) rubbing out as possible, if that's possible. I've spent a considerable amount of time and care crafting this African Mahogany Side Table, and would like to get the best finish possible on it.

Also, is there a STICKY post that includes a table of acronyms? Sure would be nice for newbies like me to have quick access to terms like BLO, BORG, Pre-Cat, etc. ;)

[Don Eddard]

I've seen recommendations to use dewaxed shellac between dissimilar finishes if you're in doubt about how well they'll adhere.

If you use the search function here and search for "acronym" you'll find this thread, among others:

http://sawmillcreek.org/showthread.php?t=78002&highlight=acronym

Making it a sticky would be handy.

[Steve Schoene]

Applying lacquer over varnish isn't a good idea because the solvents in the lacquer can attack the varnish causing it to lift.

If you have a couple of coats of varnish on there now, I think you rubbing out is going to be your best option. You could have begun with lacquer and perhaps if you are sufficiently skilled get a finish "off the gun" that you could live with, but even sprayed lacquer can look better with a little rubbing out.

[Sal Giambruno]

Thanks guys, that helps. I think I'll just stick with the varnish. I think using shellac in between layers would violate the hard-on-soft rule, and as you point out Steve, the lacquer might attack the varnish.

I sprayed a sample board (4"x6") that I had laying around that already had a couple coats of varnish on it with water-borne PolyCrylic today. It came out surprisingly well (clear, shiny, flat), but there are a few extremely slight "swells" in the finish overall (you can only see them under a very narrow and specific lighting angle), and a couple or three swells that are a bit more prominent, but no bigger than the size of a lentil, though not necessarily round. This might be due to some leftover wax that I didn't clean off well enough, or residual lacquer in my spraygun, even though I cleaned with Lacquer thinner, followed with Alcohol, and then water. Still, this tells me nothing about the long-term durability of these layered finishes.

The defects may also have been due to finish contamination from the spraygun. I came to discover that my gland nut was loose after only having used the gun a couple of times briefly. It started dripping from the trigger area when I first started spraying today. When I took the gun apart this evening for a thorough cleaning and inspection (first time ever), I discovered a couple of metal shavings about 3/8" in length stuck to the inside walls of the needle bore (probably from the CNC milling). I thought it was a pubic hair at first when sighting down the bore with a flashlight, but once I got it out learned the true nature of this foreign object.

I was dismayed to see this lack of quality control in a $900+ piece of equipment. In retrospect, the gun itself doesn't seem to be manufactured with the finest materials or tolerances. The chrome isn't very thick or consistently polished and other spot-welded or pressure fit parts certainly don't take my breath away. I don't have anything else to compare it to, but I usually get a top-notch product when I spend this kind of money on items of similar value/utility. I expected more from Turbinaire.

[Jim Becker]

Sal, using dewaxed shellac as a barrier coat doesn't violate any "rule"...and is quite common.

[Sal Giambruno]

Well that's good to know. I've gone through about 1-1/2 quarts of Zinnser Clear and Dewaxed Sealer Shellacs and have since been extremely leary about using shellac in general on anything because of the less than satisfactory results I've had, both in the final finish and the difficulty of rubbing out - it takes this shellac a day or twp before it really is hard enough to rub, even when applied under ideal conditions (low RH, average-high temperatures. The date on the can says it's fresh, but I just think it may be an inferior product. I'm going to give Target's Shellac Sealer a go, particularly to give some color to the waterborne finishes I am going to be learning to use.

Thanks.

[Jim Becker]

Sal, perhaps the problem is that Shellac is "different" and you don't treat it the same way you treat other finishes. Shellac is something you need to "learn" how to work with...it's not your local borg's varnish. ;) And a "day or two" before you can rub isn't that long. It's really in your best interest to wait several weeks before "finishing the finish" for most finishes so they can fully cure. The only reason you can rub out shellac and solvent based lacquer so soon is that they are evaporative finishes...but even then, waiting is a good idea so that ALL the solvents can flash off completely.

As a barrier coat, spraying is the way to go. Either with your own gear or just from the $6.99 spray bombs. (which are also de-waxed) You only need a thin coat.