I’m finishing my first major piece of furniture, a cherry coffee table. It’s a tung oil and mineral spirits base, with four build coats of pure tung oil that I’ve sanded in with 600 grit wet/dry sandpaper. The last coat is drying now, and the top coat will be satin poly. Before I commit to this final step, I’d just like to double check with the voices of experience: an oil-based polyurethane top coat *is* compatible with the tung oil base — right?
And a related question: I’m in Colorado. I’m finishing in my garage. Avg temp right now (in the garage) is about 70 deg F, and the relative humidity is about 30%. How long would you let the tung oil cure before adding the poly topcoat?
Thanks! …and somebody tell me that someday I won’t be a nervous wreck as soon as I start applying a finish to a project!
-M.
Replies
Mark, Relax. the shakes will go away as it becomes increasingly difficult to discover mistakes you haven't already made at least once before.
There shouldn't be any compatibility problems with oil based poly over tung oil...unless you're working with one of those "fruit cocktail" tung-varnish combos...and even then, only if there are styrene based resins (or wax) involed in the formulation of it and/or the top coat poly...BUT, you can skip the pitfalls of this particular learning experience if you test your finishing sequence on a scrap first...Of course, that's only if you want to do it the easy way. Personally, it's been my experience that if you just go ahead and make the mistake, in the long run it vastly improves lesson retention. :O)
The environmental conditions you suggest: 70 degrees F and 30% humidity, should be in-range for the curing of most finishes. Slightly higher temperatures (and oddly enough, higher humidity in the case of some finishes) might hasten drying time a little, but I don't think you have a problem.
Edited 9/30/2003 11:42:29 PM ET by Jon Arno
Thank you gentlemen!
Just out of curiousity (and a desire to avoid learning a lesson the hard way), what happens if you apply a top coat before an undercoat is cured?
-M.
>>"what happens if you apply a top coat before an undercoat is cured?"<<
Quite often nothing. Especially if the solvents of the two finishes are totally unreactive (incompatible)...as in oil based varnish over shellac. Most finishes take a rather lengthy time to totally cure anyway, so it's the rare occasion when you don't actually apply a top coat while the sealer coat still retails trace amounts of its solvent. The problems with premature application of top coats are usually more physical than chemical. In other words, you risk moving the sealer coat around, or the sealer coat's solvents might bubble up through the top coat, if they have no other means of escape, while they are still plentiful. Also, if a top coat hardens while a sealer coat is still slightly plastic (gummy or still shinking) it may cause the top coat to fracture (alligator.)
HOWEVER, when working with oil based poly over an oil based sealer, you can run into some additional problems. It's best to give the sealer some serious hardening time, i.e., well beyond its tack point...or in other words, dry to the touch and giving off no strong solvent odors. This is because the poly varnishes "polymerize"...Meaning, the synthetic resins in them form long molecular chains as their vehicles (the solvents) discipate. You can interfere with the polymerization by introducing solvents (distillates) that aren't precisely compatible. In fact, once cured, the polymerized resins are impervious to their original solvents...so, it's even critical to maintain the between-coat schedules specified for any given poly, if you intend to apply more than one coat of it. Each succeeding coat has to "bite" into the previous coat before the preceeding coat chemically locks up.
For a more technical explanation, you'll have to get with an Organic Chemist. I don't like poly varnishes, both because they are so miserably hard to rub out between coats...and also because their chemistry is so mystic. I only use them on heavy-wear table tops. And even then begrudgingly so.
Edited 10/1/2003 1:55:49 AM ET by Jon Arno
If it is 100% tung oil I would wait about 5 days before putting on the poly. But the best test is your nose, if you can smell the tung oil it is not cured.
Scott T.
Man, you're done. There's no need to top the thing with poly. Let the oil cure well and then wax it. Provide coasters if there is concern about 'protection.'
If you absolutely want a varnish finish then next time go straight to the varnish and skip the oil.
For this project, and in my opinion, poly is going to ruin what must be a beautiful finish already - multiple coats of tung oil carefully sanded in.
Keep it simple.
Mark, I'm not sure if this is your first time using polyurethane or just the first time on a project of such importance, but...
...have you thought about using a wipe-on poly instead of a standard brush-on formula? (Wipe-on is really just a thinned version of the brush-on.) You'll need to apply more coats to get the same protection, since each coat is thinner than when you brush it on, but you'll gain a little bit more control and avoid sags, drips, bumps, etc.
David
"The world that was not made is not won by what is done" -- Mundaka Upanishad
It is my first time using poly. I looked for wipe-on poly at my local home improvement store (not HD), but they didn't have any.
-M.
Mark--As suggested above, cut your brushable poly with 50% mineral spirts, and you have wiping varnish.
I agree with Crunk. However, if you are going to use poly, the wipe on method is best. Re: the drying of your oil. A nose is an okay test. Another is to take some steel wool to your piece. If it makes fine powder as opposed to gumming up, I'd say it's dry/cured.
Scott
Being a REAL novice here, however, I too am wondering what the purpose of the layers of carefully hand-rubbed/sanded tung oil is if you're going to coat it with poly??? Does it give a different color/depth/??? to the final finish. I thought if you did the tung oil thing right you didn't need the poly. Look forward to hearing from you guys who have been at this for a "while" longer than I.
Robin and Molly & Sadie (the Wonder labs) "Mom, wonder what we'd get if we just gave it a really good 'licked' finish??"It's football time in Tennessee! Go Vols!!!!
Rob, if you're going to apply a film (varnish) type top coat, I don't think it's necessary to go through all of the usual build-up coats of the penetrating oil first...as you might do if the oil was to be the only finish. However, on some woods (cherry for example, and especially curly cherry) a pre coat of penetrating oil does seem to accent the figure and bring out a nice contrast. If you go straight to the film coat, especially if it's a fast drying shellac or lacquer, you don't seem to get quite the depth.
And a pre coat of oil will definitely give you a different color than if you went straight to lacquer or poly or shellac.
I guess that's why it's always good to test your colors if you're not sure.....
Scott
OK, I'm starting to panic.
I brushed on a coat of poly (Zar, Interior Polyurethane) after my fourth coat of oil had cured for about three days -- I could detect no hint of solvent smell before I started the poly, and a pretty firm wiping with a clean finishing rag brought up zero detectable oil.
After nearly 36 hours, the finish is still tacky in places and looks just horrible. It's mottled, like it soaked into the wood in some places but not in others. I'm beginning to feel like I'm facing a disaster.
Or, am I panicking for nothing? As noted in my first message, this is the first time I've used polyurethane. Is this behavior normal? Does it typically take more than one coat of poly to achieve a good result?
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