failed finishing experiment: why?

Re: failed finishing experiment: why?

Sam Simpson

>Hi Y'all,

Bob, the primary culprit is the old can of general finish.

Oil finishes absorb oxygen as a way of creating a chemical reaction for the polymerization process. This action continues even while it is in the can. In a thin film state the oil will absorb oxygen readily and continue on to a dry and cured state. but in the can it will absorb whatever oxygen is available, which will result in a thickened state rather than fully cured. Oil finish that has already been exposed to oxygen by being resealed in a can will not dry to the same fully cured state that fresh oil will. This is because stopping and starting the polymerization process will produce incomplete molecular linkage.

Thinning and adding more oil solves nothing. You will just end up with a streaky mess. but you already know that. :)

Regards Sam Simpson.

Re: failed finishing experiment: why?

Robert Savit

>Well, OK, but the other piece of data is that just adding solvent (mineral spirits) did allow me to use the old finish with success. It was adding the tung oil that seemed to create the problem. So if the problem really is associated with the partially polymerized urethane oil, it must be some issue having to do with the mixture of the paritally polymerized urethane oil and the unpolymerized tung oil. Is there some well known problem here?

Thanks,

Bob

Re: failed finishing experiment: why?

Russ Fairfield

>Mixing a fresh oil finish with one that is over-the-hill never works. The result will be what you got from doing that.

Sometimes we can still use a oil finish that has started to age, but it will take longer to dry. A lot depends on how much it has aged. There is always the risk that it will never dry.

We can add thinner to any oil finish, fresh or over-the-hill, to make it easier to spread; and it will probably cure faster because there are fewer solids on the wood. The missing factor is always how long did it take to cure compared to a new finish. If the ageing finish is far enough over-the-hill, there is no amount of thinner of any kind that will salvage it.

Bottom line, when a finish starts to "gel" in the can, throw it out. Yes, you may be able to get away with using the aging finish awhile longer; but if it does cure, you just got lucky. It imay not be something that is repeatable.

I have had this happen.....

TDaniel

>Robert, I have had the same problem several times. The first time I assumed my Tung Oil was old so I threw it away. The next time I got the dull, foggy finish I thought it was due to using Turps instead of MS. There seems to be a problem when I add Tung Oil to Waterlox; sometimes it works and sometimes it doesn't and I don't have a clue why.

TD

Re: I have had this happen.....

Sam Simpson

>Hi Y'all,

OK, let's see if I can come up with an analogy for you that doesn't make me sound like a total racist.

Think of the crosslinking of molecules as being very much like children holding hands in a playground. The social scientists amongst us will have noticed that people tend to gather together in like kind.

If a teacher announced to a gathering of several hundred kids, that she wanted them all to start holding hands with each other as soon as she blew a whistle, there would be scurrying and confusion as neighbors linked hand. Maybe even a little heat would be created:) This linking of hands would continue until all the links were made. Without any noticeable grouping of like kinds. But if she said I want you to stop linking hands as soon as I blow a second whistle, and start and stop again every time you hear a whistle, you would soon find that, given time to think about it, there would be a gathering of like kind groups. Mini gatherings of friends all interspersed throughout the larger group. With the result being diferent every time.

The same sort of thing is at work when you turn on and off the drying cycle of finishes. Especially mixed or adulterated finishes.

Have you ever seen a finish that had a notice "do not thin" on the can. This warning is for very much the same reason. The balance of components within a soup of finishing chemicals is sometimes so finely balanced for performance, that even thinning will affect the results in drying.

Many of the one-third mixes that are advocated for amateur finishing are perfectly sound mixes, but it is essential that FRESH product be used in all cases.

Buy small quantities and throw away the fire hazard rather than store it for future use.

Regards Sam Simpson.

A good analogy

Russ Fairfield

>I have never heard that one before, but it does describe what is happening.

Re: failed finishing experiment: why?

Robert Savit

>Thanks to all for enlightening comments, particularly, Sam's analogy. Along the same lines, I wonder if the issue is clumping of size. I could imagine that when a finish starts to polymerize, it forms small clumps of linked molecules. If you then mix it with an unpolymerized finish, then, as the further polymerization proceeds, you are now linking not just molecules to each other, but a mixture of molecules and clumps of molecules. This heterogeneity in element size could easily result in a more random reflection of light leading to a dull finish. This might also explain why thinning with solvent, without the addition of other, unpolymerized finishes sometimes works: If the distribution of clumps in the partially polymerized finish is homogeneous enough, then the final finish could still look reasonably good since the elements are mostly similar sizes. But if the distribution of clumps in the partially polymerized finish is highly heterogeneous, then thinning won't work.

I have to add that, although I am scientifically inclined, I have always hated chemistry.

And still do.

Cheers,

Bob Savit

By Jove - I think he's got it! :)

Sam Simpson

>

Re: failed finishing experiment: why?

Joe Millsap

>I have been using General finishes for many years and when the finish starts to thicken throw it away. To keep finish fresh i use large mouth fruit jars and the wife's seal a meal and pull a vacuum in the jar and it will seal and keep forever. It works very good and will work on all oil base and polyurethane finishes.

Re: failed finishing experiment: why?

Arch

>Hi Bob, Good question with very good responses.

Permit a mild disagreement. I don't think of your negative experimental results as a failure. You wanted to know if a certain finish would work. You found out and we all learned from your experiment. Arch