Epoxy over shellac?

Re: Epoxy over shellac? Yes.

David Barnett - SW Florida

>I'd recommend a dewaxed shellac, however, such as Seal-Coat. If you're looking for greater depth, you can even use a good quality drying oil beneath the shellac to increase chatoyance. If you're concerned with blotching, you can spit shellac the cherry first to regulate the speed of oil penetration, wiping it down when you reach the limits of your comfort zone. Allow the oil layer to dry thoroughly, at any rate. Apply light coats of shellac to seal the oil layer, then apply your epoxy. You should, of course, test this on scrap but I foresee no problems. BTW, there are some epoxies that age more gracefully than others.

Re: Epoxy aging problems?

Dave Mount

>Thanks for the info Dave.

Re: "aging" of epoxy, are you talking about something other than UV-induced breakdown? I was planning to use West Systems 105 resin with a 207 hardener, which is what I've used on strip canoes. I was going to top that with a coat of poly provide a little UV screen. Am I headed for trouble?

There are lots of "bar top" epoxy finishes out there, but from what I've read, these are geared toward the 1/4" plastic build you often see in bars and restaurants, which I don't want.

Thanks.

Dave

West is excellent. I love the stuff.

David Barnett - SW Florida

>

Re: Epoxy aging problems?

ThomD

>I wouldn't do the epoxy over shellac thing though. You are basically contaminating your bond to the wood. Given your epoxy is hard and your wood is soft, anything that hits the surface hard may dent and spall the surface finish. This may happen even if your bond is direct. Both cherry and epoxy are pretty light sensitive. So while cherry often has a wonderful sensitive colour when fresh, it will darken a lot and the epoxy will darken also. Within 10 years the end result is a really dark finish. I am sititng here typing on a cherry table and looking at a walnut table next to a cheryr bookcase, they are all pretty close to the same colour, I think a passer by would assume they were the same wood. I am sceptical your more subtle finish elements will survive long. So I would probably approach the finish either going with the epoxy direct, or using standard repairable finishes, depending on how booze soaked you expect the surface to become. West is my favorite epoxy, but not the most chrystal clear of the available brands.

Interestingly, shelac fortified with drying walnut oil is pretty resistant to water and alcohol.

Re: Epoxy over shellac? NO!!!

Earl Kelly

>You cannot apply Epoxy over any kind of non catalyzed film finish. Epoxy, like polyester heats up as it cures, actually can get hot to the touch. Placing epoxy over shellac is asking for trouble.

The epoxy should give you all the depth of finish you need, after all, it will literally soak into the wood. Epoxy can be top coated and some are designed to be.

Re: Epoxy over shellac? NO!!!

ThomD

>It gives off heat, but it won't build up any significant heat in a thin coating. It's still a bad idea.

Re: Epoxy over shellac? NO!!!

William Duffield, on the Cohansey

>The curing of epoxy is an exothermic reaction, and in a pot with a large volume to surface area ratio, it can get very hot, and even boil (DAMHIKT). But, when spread out into a thin surface film (or even in a thin glue joint), the heat generated by the reaction is very rapidly carried away by radiation, conduction and convection, and the temperature rise is insignificant. Sometimes it is even necessary to either heat the shop or the items being bonded to achieve an acceptably short curing time.

Gee, it just must be my imagination...

David Barnett - SW Florida

>but I use shellac and epoxy together for dopping stones; the shellac for non-heat sensitive and the epoxy as a 'cold' dop. In fact, these two different 'stickums' reside together in layers and I've NEVER had one or the other fail, even when dopping on a lathe while applying considerable side thrust. Just me? Tell it all the others who do the same thing. And I HAVE used epoxy with shellac in the same way as I recommended as a waterproof finish. Guess what, it's a thin film when applied. Most of the pot heat, as Wm. says, is disippated While I'd never use epoxy for its 'aesthetic', I assumed the querant had his reasons. Another nice thing, epoxy properly chosen and applied flexes nicely with shellac. Sometimes I gotta wonder... heh.

I will also add...

David Barnett - SW Florida

>I'd be very afraid to shake the hand of any man who could separate the bar of shellac and epoxy that I'm holding without by just pulling at the ends, heh. So, YES!!! I'd still say you can. The universe has not apparently changed the rules since I posted my first response. You mileage, of course, must vary greatly.

Examples of use...

David Barnett - SW Florida

>A couple examples of epoxies and catalytics over shellac (from lutherie sources) where it is fairly common.

http://home.asparagine.net/ant/blog/?page_id=23

and:

http://gicl.cs.drexel.edu/people/sevy/luthierie/guitarmaking_guide/rosette_inlaying/rosette_inlaying.html

"In preparation for gluing, the groove is next sprayed with a couple coats of shellac. The purpose of the shellac is to seal the end-grain on the edges of the groove near the centerline of the top. If this isn'e done, the epoxy used to glue the rosette will wick into the endgrain, discoloring the spruce near the soundhole. You don't have to be careful when spraying the shellac, as the overspray on the top will disappear during finishing. (However, if you don't use the shellac, the epoxy staining doesn't disappear even under the finish - discovered this the hard way...)"

http://www.shopmaninc.com/pdf/kkote.pdf (between absorbent items and 'poured' epoxy finishes.

I've also used epoxy over shellac on metals and who knows what else. Furthermore, if you use epoxy with already warm shellac, good luck getting it to delaminate.

Re: Gee, it just must be my imagination...

Earl Kelly

>I don't doubt your use of shellac and epoxy. But I still stand by my rules I was taught 35 yrs ago, when I built surfboards and repaired boats. Never apply a epoxy or polyester over a film finish. Are you laying down a coat or two of shellac and getting a dry film build of more than a couple of mills. And then applying at least 2 good coats of epoxy with no problems? If so more power to you!!! I would not do it.

Re: Epoxy over shellac? NO!!!

Earl Kelly

>William, I know epoxy is exothermic, but it gets warm enough to expand the air in trapped air pockets. And on occaision I've gotten polyester hot enough to catch fire in the container. Usually happens when your in a hurry and can't wait for the normal firing off time. Polyester is much easier than epoxy to make it boil and smoke.

Re: I will also add...

Earl Kelly

>Here's pic of a piece in Epoxy, wet sanded and buffed. Solid wood CKT table shaped and sanded. This is what I do with epoxy.

Earl, you do beautiful work!

David Barnett - SW Florida

>And I have no doubt of your expertise in applying two-part finishes. That top looks like you could use it for a surface plate. However, I have to take exception to your categorical "NO!!!" against doing what I know can be done, have done myself, know others who do so routinely, and can point to epoxy finish manufacturer's instructions that recommend it.*

It is quite possible you aren't fully aware of shellac's industrial uses, but shellac is widely used as a primer for plastic parts and films, is added to catalytic moulding and modeling compounds and is a "crosslinking agent for PU-Coating, PU-Forms, epoxy resins" and is a component of several epoxy glue systems (sorry, I've lost attribution for the quoted text"). It is also combined with other resins to enhance the ability of flexographic printing inks to adhere to less than perfect substrates. Hot undissolved shellac or dried and reheated shellac coatings (spirit deposition) have unusual abilities to seal, adhere, and interlock with otherwise unreceptive substances, such as diamond, where it was derigeur for setters over decades. Shellac is used to slow and control epoxy surface film penetration in leather weatherproofing, allowing it to remain flexible. It is also used with epoxy in several nonphenolic circuit board materials.

While I'm completely agree that epoxy/polyethylene will fail over several film finishes, especially where there is sufficient crosslinking in as little as two days, and that ideally one would use an 'erased' shellac (where all traces of alcohol are depleted to prevent esterification, which is why shellac is being retried by the pharmaceutical industry), this is really not so critical for wood finishing.

At any rate, while I can appreciate your disinclination to change your technique after 35 years experience, I felt shouting "NO!!!" to be a overly strident and prohibitive. I can tell you I've bent so very many rules in quite a few crafts and while I respect heuristic generalisations as good guidelines for practical success and safety (often manufacturers have strong reasons for their recommendations, such as profit and litigation avoidance), I would've missed out on a lot of discoveries and new paths had I not colored outside the lines now and then.

.

*http://www.shopmaninc.com/pdf/kkote.pdf (between absorbent items and 'poured' epoxy finishes.

I've been shellac'd

Earl Kelly

>Well, David, I have to step back from my NO in my original post. Maybe I should have said I don't recommend. Evidently I don't know enough about shellac. I bow to your superior knowledge. Never intended to stir up such a mess.

Thank you for the complement on the table finish. Hope you have continued sucess using your combination of finishing techniques.

Is your email addy indicative of your vocation?