Raw/Pure Tung Oil finish - seeping issues

Question

PLEASE: IF responding to this post, do NOT reply unless you know the difference between Raw/Pure/Unadulterated Tung Oil, Polymerized Tung Oil, and what is marketed as "Tung Oil Finish" -- or tell me why Raw/Pure Tung Oil is bad, and other finished are better, or to sand everything off and use Poly or Varnish instead. I have used Raw/Pure Tung Oil (100% Pure Raw Tung Oil from Real Milk Paint company) MANY times with great success, and this is my first problem. I am mid-way through this project, and am only interested in replies from those with EXPERIENCE with Raw Tung Oil that address what I should do NOW to fix it.

I'm finishing a table with Acacia/Rubberwood butcher block type top and it seems that the large pores of the Acacia are giving me trouble.

I followed the usual steps I always do: flooding with 50/50 Tung/Citrus Solvent for the first application, 75/25 for the 2nd, and about 85/15 or 90/10 for the third.

The second and third coats I also wet-sanded the oil in using 0000 Steel Wool and then wiped the sludge away thoroughly.

Between each coat, I wiped all excess oil off and waited for at least 4 days in between each. It took much longer to dry than I've ever experienced. But I know each coat was DRY (not of course CURED) -- because between each, I sanded lightly with either 220 sandpaper or the Steel Wool, and the dust was a fine white powder - no gummy residue, indicating it was dry enough to add another coat.

(I live in FL, but I have the table inside where it is normally air-conditioned to 76 degrees, so the humidity is NOT high. To aid in the drying I turned the AC down to 72 so the air would be even drier.)

After each coat, I noticed that the oil was continually seeping out of the pores, so I was wiping it off about every 15-20 minutes for the first 8-12 hours, and then several times a day for the next 2-3 days. Again, I've never experienced this before, and I'm sure it has to do with the large pores of the Acacia.

Still, the first 2 coats went on fine and were completely dry before applying the next coat.

On the third application, I kept wiping and buffing as usual, and all was fine until the third morning when I checked it, oil had seeped out overnight and DRIED, so now there are dozens of tiny spots that are glossier than the rest of the table. They're not white -- just glossier than the rest of the table -- and you can only see them if you look at the table close to eye level in certain light, but the table is not as smooth where the dried oil is.

In an attempt to fix, I tried sanding with 0000 Steel Wool and then wiping down with pure Citrus Solvent - and then it was like a dam broke, and oil was seeping out of the pores all over! I was able to wipe that away, but the previously dried spots are still there - the steel wool and Citrus Solvent didn't touch them. Also, the Citrus Solvent seemed to dry out the rest of the table and make it duller.

On a small section of the table, I tried using a 3M white Final Finishing Pad (which is like a Scotchbrite "scrubby") and a bit of Tung to see if that would break up the dried spots, but that did nothing.

My plan was to do 1-2 more final coats, this time of 100% pure Tung - no Citrus Solvent. However, I don't want to do that until I know the best way to proceed.

Should I wait for several weeks to make sure the oil IN the pores is fully cured, and then sand and do a coat of pure Tung Oil, wet-sanding in various steps from medium to fine, to try to get rid of the spots?

Or, if I wait for it to thoroughly dry, and sand, and then just apply the pure Tung, will the Tung eventually build up so the rest of the table is as glossy as the spots, so it won't be noticed?

Or, if I wait for several weeks and apply Polymerized Tung -- which has a glossier finish - will that bring the rest of the table to the level of gloss of the pores, making it uniform? I've never used Polymerized Tung before.

I really do not want to sand everything off and start over.

Any advice would be appreciated!

Many thanks,

Answer 1

I came across this question while researching tung oil characteristics. The question was posted a year ago, so the original need is no longer relevant, but for others with similar questions it may be of use if I post what I learned.

Tung oil does not "dry" by evaporation (like water or organic solvents). It cures by polymerization. The curing process is facilitated by oxygen. Tung oil on the wood surface therefore polymerizes relatively quickly compared to tung oil that has penetrated into the wood, where it is in a relatively oxygen-poor environment.

Another useful fact is that tung oil polymerization can be catalyzed using a "drying agent", which is typically a metal-salt in a form that allows it to form an emulsion when mixed into a non-polar substance (ie. most oils). A tung oil combined with such a catalyst will cure faster and the cure will be less dependent on exposure to oxygen in the air. An example of a drying agent is manganese-acetyl-acetonate. Cobalt-compounds are also commonly used. Other metal compounds can be used, but they tend to be more toxic or have strong coloration.

So, based on what we know about the problem described, it appears that slow curing of oil that had penetrated deep into the wood allowed it to seep back out over time. The structure of the specific wood used may have played a role in allowing deep penetration of the oil. The seeped oil that reached the surface was exposed to relatively high levels of oxygen, and cured to hard polymer that was resistant to solvents.

Solutions could include: 1. Thinner coats with ample "drying" time between coats. 2. Adding a "drying" agent to the tung oil.

Answer 2

I've used Pure Tung Oil on both Hickory and Beechwood. My research showed that a thinner could be used on the first coat only, in order to get better penetration. I made the same mistake on my 3rd coat (applying too much because I thought the Beechwood would soak it all up) and the Beechwood continued to leach for about 24 hours. I kept wiping it down with a rough cloth and it seemed ok to cont.. It is recommended to do 3 coats (waiting at least 24 hours between each), then wait a month (or more) before applying a final coating. This should last 2-3 years on an axe handle. Using thinner on every coating was not recommended. A final finish with SPAR Urethane can be done since the SPAR will cut through the Tung oil on the surface. Bring them inside to cure in the cold months, the heat and lower humidity from your heater will cure it much faster than leaving it in the garage.