BLO and pre-finished ply

[millwab]

Scrollers,
I tried something new again, and have another question. I cut a portrait out of ply that was pre-finished on one side (in my case the back side). I tried dipping the finished piece in BLO which I placed on paper towels for a minute or two after draining off. The front side is fine (which, I guess, is the important part) but the back side that was pre-finished obviously didn't absorb the oil. Four days later there is still a tackiness to it. Will it eventually dry, or am I going to have to let the backer (poster board) get 'glued on' when I put it in a frame? Any thoughts?

Bruce

[workin for wood]

I'm almost afraid to answer because last time it kicked up a huge controversy. although I know it wasn't all because of me. I'm going to say that it will dry eventually. Just a hunch, may take awhile though, I'd try waiting it out at least a week. One time I sprayed poly on ebony and i thought it would never dry, but it did after about a month ! Now I know to wipe ebony with mineral spirits first to remove the oil. I'm always learning from my mistakes when finishing, and now I see you are too.

[Marcel in Longueuil]

Bruce,

Have you thought of "helping" it along with a blow dryer?

Just a thought I got reading your post, not sure it would work.

If you have a cutoff to try it on, might be worth it

Regards,
Marcel

[lucky788scroller]

Did you try wiping it with mineral spirits , and put it where it gets good air flow around it. You dont want heat to cook the finish dry, because under the surface it will not be cured.Try wiping with mineral spirits, and placing it by a fan for a few hours, maybe a day or more. Dale

[Jediscroller]

I'm with Dale on this one. I think mineral spirits is the way to go. BLO is meant to soak into the wood and the finish would prevent that from happening. You may have to do it a few times to remove all of the BLO.

[MacS]

Not a big deal...

Evidently, the coating on the wood is impevious, and the BLO is not going to be a good bond even if it takes a month to dry.

I would remove it, if the mineral spirits don' take it completely off, you could use lacquer thinner that will do the trick.

Don't wait, do it now.

Mac

[Marcel in Longueuil]

Dale, Kevin you have piqued my curiosity.

My question is: If the hot air cooks the outside first letting the inside not cured as you mention, then how does it cure naturally?

From the inside out?

Isn't it curing in reaction to the air (sort of oxydising)? And isn't that why you can have something dry to the touch but not fully cured before a week?

I agree that if the heat is applied too close to the source it isn't good and that is why I did not suggest using a heat gun (they use the heat to strip the finish by softening it) but I would have thought using a blow dryer with warm air (not hot) could have been done.

The initial question wasn't to obtain a fully cured finish, but one that was dry enough to mount in a frame without sticking to a poster board, and since it's not showing (it's the back of the piece) would it realy matter if it isn't perfectly bonded to the underlying coat of stain?.

I'm not saying you are wrong in your statement or that I'm right: I just wish to learn more since you & Jeff have me doubting myself in my initial answer. You may know something I don't that justifies your negating what I said.

Regards,
Marcel

[Jediscroller]

Marcel,
I don't know about the heat part. You do have a valid point about it being the back. My reference was in regards to taking the finish off. My concern there would be that if left on it may leech (leach?) out through the frets. I have had this happen to me with oily woods where the oil finish never completely dried (lack of a better description). I ended up with little globs of sticky stuff all over the piece.
This part is purely a guess on my part, I'm guessing that the finish does actually dry from the inside out as it reacts with the air. I think if it were the other way around, the problem that I described above would happen with every piece that was finished with oil. I could be wrong though. Wouldn't be the first time and I'm quite sure it won't be the last.

[lucky788scroller]

My thinking is that naturally curing is by the moisture (thinking of the proper words here, so bear with me) dissipating over time, slowly, and evenly. If cooked with a hairdryer, the surface may dry,or at least be dry to the touch, and the underlying finish still gooey , which would then take a long time for the moisture to dissipate.All finishes that I can think of have an initial dry time (or dry to the touch), and a fully cured time, which is much longer. Just because its dry to the touch, or the surface appears dry, doesnt mean that once the backer is applied, the trapped ,uncured finish wont cause the backer to get stuck to the finish. In fact, I'm almost willing to bet that it would stick to it, unless its been the full however many days the lable read, and applied correctly.I am no finishing expert by any means either,in fact, I didnt even have time to finish my lunch today!
By wiping it with mineral spirits, you should be able to get off some of what currently isnt drying right, and it will also thin what is on there, hopefully enough to help it cure properly.I know for fact that method works great on polyurethanes, I cant imagine what it wouldnt on BLO. Hopefully that made sense , if not, tough,because I dont know how to explain what I mean any better! Dale

[Marcel in Longueuil]

Actually it makes a lot of sense Dale,

When drying maybe it dries up on the surface first (creating a dry to the touch surface) but the surface probably remains porous enough for the finish to cure ( a "Deep" dry of sort) letting the moisture or solvents (or whatever chemicals) evaporate


I think that you're right that with time it would stick to the backer board.

And Kevin, I see the globs leeching to the front could definitely happen.

Thanks for sharing that.
Marcel

[MacS]

Using a Hair Blower

Dale,

Your right, hair blowers are used all the time by professionals servicemen and in finishing shops to speed up the drying..not the curing. BLO dries from the top down, to prove this look at a used can of BLO the skin is on the top, so, it had to dry from the top down.

Dale, I personally would remove the BLO, even if it dries on that pre-finish plywood, as there will be a poor bond for the backer board.

To attach the backer board it maybe nessacary to use CA glue or an epoxy glue to get good bonding.

Mac

[sawdustus]

sawdustus of hiawatha

You are all partly correct in your answers. Some finishes cure from the top down and some from the bottom up. The answer depends on the chemical nature of the finish. Is it evaporative, reactive or coalescing?

The following information was extracted from Bob Flexner's book Understanding Wood Finishes 1994, Chapter 7 pp. 135-142

Evaporative - the solvent evaporates and leaves the dissolved solute behind to be redissolved if you wipe with more of the solvent. Think shellac or lacquer. Cures from the bottom up.
Reactive - the solute (resin) molecules actually react chemically as the solvent evaporates. These are generally professional shop finishes and are the ones that they heat up to cure. For us, think varnish when after it dries, you have to sand to get another coat to stick. Cures from the top down
Coalescing - typically water based finishes and white and yellow glues cure this way. More complicated than I want to try to summarize.

I hope this is not too technical but I was a chemistry-earthscience-general science teacher in a previous life.

[Jediscroller]

George,
Thanks for the info. I have several of Flexner's books, but this isn't one of them....I know what I'm buying this weekend.

[MacS]

Another descrption of coatings curing

Oil finishes are also a reactive coating, once any of the reactive coating cures, all the following coats that you apply will become a seperate layer. The more coatings you apply the more layers you will end up with.

Whereas, in evaporative coatings, no matter how many coats you apply, you end up with only one layer because, evaporative coatings dissolve all previous coats to become one layer again.

[NC Scroller]

Try using the sun

I know it is winter now in Ohio (been in the 70's here all week in NC ) but if the temperature is above freezing try laying it outside in direct sunlight. Good luck....Scott