We have varnished plywood floors and want to glue and screw 3/8″ plywood to the varnished plywood so we can then lay ceramic tile. What glue would you recommend?
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A quality latex modified thinset.
Fine
Good grief , did you shovel the wrong driveway the first time?
Snow, what's that?
(Greetings from tropical southern Minnesota, where we got about a half an inch today.)
I think CBU's have very limited positive attributes as a ceramic floor underlayment material. In fact, I'm surprised the box stores still sell as much as they do. I chalk it up to a misconception of folks thinking you need CBU's to properly bond ceramic tile. You don't. In fact, Schluter long ago illustrated that ceramic can be successfully bonded to polyethylene (a moisture barrier) via heat welded polypropylene fibers and modern thinset formulations a long time ago. They wouldn't be selling millions and millions of square feet of Kerdi every year if it weren't so. Schluter's Ditra product also demonstrates how ceramic tile can be bonded to polyethylene (again a moisture barrier) with simple geometery and thinset alone--and provide reduced dead loads along with a decoupling feature from the structural substrate to boot! IMO, CBU's can't hold a candle to that.
I recall a job that we were called in to fix about 10 years ago where a customer had grout working loose from between all his kitchen and living room tiles (12x12's) that were installed by a previous remodeler only 6 monthes earlier. We removed about 800 sq. ft. of ceramic that was installed over 1/2" CBU's. The CBU's were installed over 5/8" T&G subfloor. Besides the structurally inadequate substrate, the interesting part about this job was that we were able to reuse about 99% of the removed tile. This was because less than 10 pcs had any substantial amount of thinset to speak of bonded to the back of the porcelian tiles. Most had absolutely none bonded! In part, this was because the CBU's had prematurely sucked the moisture from the thinset before it has a chance to hydrate properly and bond to both the CBU's and the porcelian. Granted, this was evidently caused by the original installers failure to pre-wet the CBU's and/or applying thinset improperly, but I find weak CBU-to-tile bonds to be a fairly common problem (in varying degrees) in many of our remodeling demolitions. I don't find the problem as much with tiles bonded to plywood with latex modified thinset. I am not saying that tile cannot be set properly over CBU's. But some "professional" installers clearly don't know how... and (IMO) it's generally not worth the extra time, effort, or dead load to specify this material.
To get back to your orginal question, if you want to use a wood glue to glue plywood on top of varnished plywood you at least need to "rough up" the varnish -- not necessarily sand it all off, but use some coarse sandpaper to make holes in it that the glue can get through. If the varnish isn't at least partially removed you'd need to use some sort of mastic instead.