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A few years ago there was a posting of a website (I think it was some sort of woodworking/forestry dept type site) where the various waterfastness properties of oils and varnishes were assessed. I recall tung oil not rating as high as polyurethane. Does anyone perhaps have the link? (Steve Dugmore)

Here is a chart taken from "Understanding Wood" by R. Bruce Hoadley that was originally published in "The Wood Handbook" by the US Department of Agriculture.  It shows Moisture-excluding effectiveness of various finished.  Others included in the chart and not shown here are acrylic and latex paints, marine enamel, pigmented varnish, etc.

Moisture-excluding effectiveness of various finishes on ponderosa pine*.

* Sapwood was initially finished and conditioned to 28*C (80*F) and 30%RH, then exposed to the same temperature at 90% RH. p. 208

Also a brief snippet regarding slowing moisture exchange:

"Although a primary objective of finishing treatments is to prevent moisture exchange, no finish is totally effective at doing so.  Given enough time, moisture will be absorbed into wood from a humid atmosphere or will escape to a dry atmosphere through any finish.  But as discussed earlier, the important role of the finish is to retard the rate of exchange enough to buffer the temporary extremes of high and low humidity.  The effectiveness of a particular finish may also be affected by the number of coats applied and the time of exposure to a different humidity level." p. 207

and:

"If there is anything worse than no moisture barrier at all, it's an uneven moisture barrier, which allows moisture to be absorbed or desorbed unequally in different areas of the wood." p. 209

As an interesting example, on a particularly long and rainy day last summer I watched a set of wraps that were color preserved with 3 coats of polyurethane spar varnish then coated with 2 dipped coats of marine spar varnish actually get slightly mottled in a few places from moisture absorbing into the silk.  There were no breeches in the varnish, and after the rod sat inside for a while the mottling disappeared.  (Chris Obuchowski)

So shellac and urethane varnish are the best. And 6 coats! I have a lot to learn.  (Hal Manas)

Not sure I'd want to see a rod with a build-up of 6 coats of spar or polyurethane on it though.  Remember, even at 6 coats, the cane is still going to absorb and release moisture.  The finish is there to protect the cane, and slow down the moisture absorption and release.  The cosmetic value is of second importance,  though it is highly regarded.  (Mark Wendt)

Thanks Chris, very interesting data.  Now I know why dipping wood into hot paraffin wax works so great to keep my reel seat blanks from checking while they dry.  (John Rupp)

I found it again in Bob Milward's book. Here is the link.  (Steve Dugmore)

That is depressing! I always thought that a good coat of wax was the best. Nothing that I use is very effective.  (Hal Manas)

Only for those who are interested in tung oil,

I couldn't testify  to the relative effectiveness of other finishes, but pure, polymerized tung oil has proven to be one of the BEST barriers against moisture absorption. The biggest problem in discussing tung oil is that we've become totally confused by marketing, and no longer know what the product really is.

The various "tung oil" finishes one sees on the shelves in paint stores actually contain little (or no) tung oil. This isn't a food product, so manufacturers aren't required to use the word with much specificity. Thus, ANY amount of tung oil in the mix allows the maker to call it tung oil. In fact (as with "Formby's Tung Oil Finish"), some products contain no tung oil whatever. Overall, these are usually very good products and can give an excellent hand-rubbed look, but they're not tung oil in any proper sense.

As to 100% "pure" tung oil, there are two varieties. The first is raw, also called "Chinatree Oil." It is refined only in the sense of removing impurities, and it gives a beautiful look to wood surfaces. In this raw state, it will eventually dry to the touch, but it continues to cure almost forever. Within a year, the surface may deteriorates, and a refresher-coat must be rubbed in. This oil is best used as an additive to certain other paint and varnish products, because on its own, it isn't particularly durable and doesn't provide a good moisture barrier.  In this,  raw  tung  oil  is  similar  to  both  raw  and so-called "boiled" linseed oil.

The second variety of "pure" tung oil is polymerized--actually, pressurized and thermalized. This means that the oil has been cooked under pressure at very high temperatures, then held there for a period of time, stripped of impurities, and cooled in stages. This oil contains 100% solids, no siccatives, reducers or other additives. (Notice that, while "boiled linseed oil" used to go though a similar process, commercial production of linseed oil hasn't involved actual boiling for over fifty years. Instead, it's now processed in various ways, with driers and reducers added to make it behave as the old, boiled variety used to.)

The viscosity of polymerized tung oil is like thick syrup, and the process has made the oil ready to complete its cure. When thinned before rubbing (at least by half), the reduced tung oil gasses off fairly quickly, leaving the polymerized oil to "dry" within a couple days, and complete its cure within a few weeks.

"Dry" is put in quote marks here because the word suggests evaporation, but tung oil does not cure through evaporation. (The reducer will evaporate, but not the oil.) Instead, it absorbs oxygen to harden its own molecular structures. Thinning with pure, gum turpentine actually serves as a partial catalyst in this process, as the turp (unlike mineral spirits) introduces resins of its own, while also "exposing" the oil to oxygen. Oxygen causes the molecules to bond at various of their sites, forming cross linked chains--completing the polymerization. (Actually, if you have fifteen or twenty years, the raw stuff will eventually do this too.)

In short, properly cooked, polymerized, tung oil leaves a flexible, leathery coat. With the right humidity, temperature and air circulation, you can rub coats one-every-other day. These will build and when cured, you're left with a hard surface film. By contrast, raw and "boiled" linseed oil can't build, and raw tung oil won't build either. The film of polymerized tung oil isn't nearly as scratch-resistant as a good alkyd or polyurethane varnish, but its protection against water vapor is second-to-none (with rubbed-on paste wax being the least effective).

Now, which tung-oil preparation did the Forest Laboratory folks use, and how did they apply the coats? We know what tung oil can do, but we have no idea how they ran their tests. There's always more to think about than what we read. And I'm sure that would include this post.  (Bill Harms)

I must say I really like tung oil as a finish. I would love to have more confidence in its waterproofness (is that a word?)

When you say "polymerized tung oil has proven to be one of the BEST barriers against moisture absorption" is that proof based on some sort of scientific testing or simply by virtue of having been used on older rods that are still with us?

I sometimes wonder if the bamboo is heat treated adequately whether waterproofness is something that is worth worrying about as much as we do. I have been fishing one of  my own rods with no finish for sometime now with no apparent  ill effects. But then I don't fish it that often.  (Steve Dugmore)

Here's an exerpt from (Russ Fairfield) a friends web page on "Tung Oil" ... While this has very little to do with bamboo rod making his take on finishing may be as old as him but I think still relevant.

FINISHING SECRETS . . .

6. Tung Oil

Pure Tung Oil provides a hard and tough surface finish that is absolutely waterproof; impervious to dust, alcohol, acetone, fruit and vegetable acids; and it doesn't darken with age like Linseed and other vegetable oils. All of these benefits come at a price - pure Tung Oil takes forever to dry, it doesn't penetrate the wood surface very well, and it is expensive when compared to other drying oils. Tung Oil is a "reactive" finish, commonly called a "drying" oil, in that it will dry and harden when exposed to air.

Everyone asks me about Tung Oil, but nobody wants to use it because Tung Oil is not a fast finish. It takes a lot of time. But, it is a simple and forgiving finish, and when done properly, its beauty is unmatched. Sometimes we try too hard to avoid the slow and simple things in our modern high-tech lives.

I have used the stuff for years and I can share what I have learned. Other oils are commonly used in finishes because they are less expensive.  Linseed, Soybean, Walnut, Sunflower, Orange, and other fruit, nut and vegetable oils are oils that make suitable finishes for wood. Linseed and Soybean Oil are most often used in commercial finishes. Although neither are a natural "drying oil", the addition of metallic drying agents make them suitable for finishing.

Pure and Polymerized Tung Oil*

The only difference between "pure" and "polymerized" Tung Oil is that the latter has been through a cooking process to partially complete the molecular cross-linking that occurs in a drying-oil. These are often referred to as "partially polymerized" oils.

While pure oil is very slow drying, the polymerized is fairly fast drying because much of the drying time has been used up in the "cooking" process. Both forms are difficult to store. After about a year or two, depending on the humidity, temperature, and exposure to light, they will start to form a film on the surface or a gummy deposit around the edges of the container, and at that time they have to be tossed out. Bloxygen or collapsing storage bottles are recommended for storage.

I know of several sources for the polymerized Tung Oil. Sutherland WellesÒ brand is available from Garrett Wade, Lee Valley Tools, or direct from the manufacturer. It is sold with various quantities of thinners, but the 50% solids ratio is recommended for our use. Add turpentine to thin it and improve penetration for the first (only) coat. After that, use it as it comes from the can. The latest price is about $34 for a quart. Visit their website here.

A less expensive source is a product from Woodworkers Supply in their proprietary J.E. Moser® brand. It is called "Polymerized Tung Oil Varnish." The high gloss mix contains 45% solids. It is an excellent product that sells for $17 per quart, half the price of the S-W® brand. Don't worry about the word "varnish" because this term is in common use for anything that develops a surface film. I used to buy a "Jasco"® brand of Tung Oil, which was polymerized, contained something like 50% solids, and was sold by True Value Hardware. It has since disappeared, but may be available as a special order item from them.

Because of the price and availability, I have also used a lot of 100% pure Tung Oil. It is slower drying, but this will be improved with thinners. I usually use Turpentine, but other thinners can also  be used.  VM&P Naphta  dries faster  than turpentine,  and 1-K Kerosene  is slower to evaporate. Tung Oil loves to be rubbed, and the more heat generated the faster it dries. This makes it a great "friction polish" for lathe finishing or hand rubbing.

The best pure Tung Oil that I have used is available from Daly's in Seattle at their Stone Way store where they mix all of their products. They will go in the back and draw a quart from the bulk supply that they are using. It is the freshest that I have ever used, and costs about $15 for a quart. I have never been able to talk to their chemist, but I believe that it is partially polymerized as well. At least it behaves that way.

Other brands that I have used are "Old Masters"® or "Hope's"®. The Old Masters has served me well over the years and I have always preferred it because the in-store stock has always been fresher.

When applying polymerized Tung Oil, I do a few things differently from the directions on the can. After sanding to 320 dry, I apply a liberal coat of Watco Liquid Finishing Wax, and then wet sand with 400-grit, wipe it off, and allow it to dry. ALL of the surface blemishes will be amplified, and the little surface wax that remains will act as a lubricant when I go back to clean them up.

Then I apply a brushed on coat of Deft Lacquer as a sanding sealer. You may want to use Park's® brand Lacquer Sanding Sealer. I avoid the commercial sealers because they contain opaque zinc-stearate fillers that can mask the grain color and pattern. After sanding to at least 600-grit, I apply a heavy coat of Deft, and immediately wipe it dry with paper towels. Then I leave it to dry for a few minutes, then buff the surface with 0000 steel wool or the gray ScotchBrite, and then repeat the application if there are any rough spots.

Then I apply the oil with a soft cloth. Polymerized is used straight from the can. Put on a thin even coat, and avoid overlaps, if possible.  DO NOT wipe it dry as instructed on the can. Let it set 24 hours, or until dry. Truly dry will take three weeks, but it will be cured enough to re-coat when it feels dry and comes up as a white powder when buffed with 0000-steel wool.

Sand after each coat with 0000-steel wool, and apply 4 or 5 coats. Leave the final coat alone. I apply as many coats a required to get the gloss that I desire, and then apply one more.

The only difference when using "pure" is that I thin it to a 4/1 ratio of thinner/oil for the first coat. For subsequent coats, I reduce the ratio of thinner to oil to 2/1 or 1/1, whichever is required for it to flow smoothly in a thin even coat without lines. Other than that, it is applied the same way as the polymerized, except that I don't worry about overlapping, and just keep rubbing until the piece is covered. It does take significantly longer to "dry." I always cure it overnight in my dryer box with the light bulb sized for about an 90° F inside temperature.

Far more info here on his web page Russ Fairfield  (Ron Hossack)