Mad Science 101

Speaking of which, …
…you’d better have a good vacuum pump. Sucking a mixture of air and mineral spirits through some cobbled-together contraption that might allow the fumes to come in contact with the electric motor might cause…well, I wouldn’t want to be anywhere nearby when it ignited!

Pressure First, Vacuum Later
I still say you should heed what the pressure treated lumber industry does. Wood is stacked into a chamber, the chamber is sealed and flooded with preservative, and then the chamber is pressurized to force the preservative into the wood fibers. At the end of the process, the preservative is drained from the cylinder for reuse and then a final vacuum is applied to pull any excess liquid out of the wood. A lot of research goes into the best means of ensuring maximum penetration, and all the major suppliers of pressure treated wood use pressure first, vacuum last.


Refrigeration service vacuum pump


You need positive pressure!
Not a vacuum. But, how about a few psi of vacuum for a week or two to suck the juices and vapors from that wood, then, pour in the watco and increase the pressure a few pounds? And I would do it outdoors. Explosions in the shop are bad. But, controlled explosions outdoors without injury or property damage are a good thing.

Regardless of what pressure
- - - Please watch out how much pressure - -

Schedule 40 pipe may be rated for 200 PSI - - Doesn’t mean it’s a good thing to do, and who knows what happens under a vacuum

I bet they are trying to get the most…

– Last Updated: Nov-15-07 7:01 PM EST –

...effect from the least amount of product used. To REALLY fill the voids, you need to remove the air FIRST. Using pressure first will shrink the air voids (think compressible bubbles), but the total volume of air within the material will return to what it was once the pressure is released. By applying a vacuum first, there will be no air expanding and taking up space later on, and thus, you can pack more preservative into the wood.

No doubt "filling all voids" isn't necessary to adequately preserve the wood, or the wood industry WOULD be applying vacuum prior to flooding the wood with preservative. You say they apply vacuum later to recover excess preservative. Sounds to me like they want to minimize the amount of preservative that gets used, not maximize the amount that remains in the wood.

Update: Turns out this is exactly the case. This is the first thing that came up for me on Google. It's a description of the process that is used when it is desired to fill voids within the wood to the greatest practical degree. This same reference did make it pretty clear that for MOST wood-preserving methods, filling air spaces within the wood with preservative is NOT a goal, but when they DO want to really pack in the most material, here's what they do:

Full-Cell Process
The full-cell (Bethel) process is used when maximum preservative retention levels are desired, such as when treating timbers with creosote for protection against marine borers. Figure 10.8-1 presents a flow diagram for the full-cell pressure treating process. In addition to creosote, the full-cell process also is used primarily with waterborne preservatives. The full-cell process steps are listed below:

1. The charge of wood is sealed in the treating cylinder, and an initial vacuum is applied for approximately half an hour to remove as much air as possible from the wood and from the cylinder;

2. The preservative, either heated or at ambient temperature depending on the system, enters thecylinder without breaking the vacuum;

3. After the cylinder is filled, the cylinder is pressurized until no more preservative will enter the wood or until the desired preservative retention is obtained;

4. At the end of the pressure period, the pressure is released, and the preservative is removed fromthe cylinder; and

5. A final vacuum may be applied to remove the excess preservative that would otherwise drip from the wood.

I’m not the brightest candle
And I’m scant on chemistry (I’ll blame it on art school) so whadda I know? But I think oxygen is required to polymerize oil finishes, the mixture near the surface will set-up first. When you force the oil mix very deeply into the wood grain as you propose I think a lot of it will “bleed” back out, and do so for a much longer period of time than it would under normal atmospheric conditions. I would think that would impede the set-up stage. I imagine you’ll be re-rubbing a pressurized finish for an extended period of time. Days? Dunno… And a thing we all need to remember about an oil finish is that no bleeding can be allowed to happen after you’ve let your attention lag – otherwise the “big mess” of sticky spots.

As to pressure and vacuum. I don’t think there’s much to grain. Taking your rails off and treating the backs seems like a very worthwhile thing to do every once in a long while… But I think if you just swab the pieces and rub as per usual using your favorite oil mix three or four times over several days you’ll have a jim-dandy finish without the hassles of messin’ around with a lot of contraptions.

…then you’d have so more time an’ energy to… …I dunno… maybe rub that Bell seam with rubbin’ compound some more… ;^D


Arkay to the Rescue!

– Last Updated: Nov-15-07 7:10 PM EST –

Wow, that's a really good point. The stuff would tend to keep leaking out for a while. After all, why does it remain a liquid while sealed in the can, but not days after application?

Something else to consider, maybe a minor point, is that after you've successully filled a lot of the void spaces within the wood (and taken Arkay's advice to keep wiping off the dripage for as long as necessary), how much have you added to the weight of the gunwales? Think about how much heavier waterlogged wood is than dry wood. I know oil is less dense than water (so oil-soaked wood won't be as heavy as waterlogged wood), but still, keeping a boat's weight down is usually considered a worthwhile goal, and wood trim is already heavier than aluminum or vinyl.

Here is a way to estimate
Weigh can of finish before using

Apply finish

weigh can afterwards

Guess scrap, drips brush residue,

Probably less than a pound overall

I would tend to disagree
A majority of the weight of an oil mix evaporates away during polymerization. That is to say the weight of the thinner and resins is significantly reduced during set-up. Only the remaining hardened resins would contribute to the overall weight of the canoe and that would be very minor. A typical oil finish would be measured in just a few grams, far less than a thick surface film such as varnish for instance. I doubt that anybody would notice much deference in weight before/after oiling in the real world. …me thinks… RK

Arkay’s right on
just about all counts. I think this ‘on’ vs. ‘in’ question about oil finishes on wood was settled years ago in an article in Fine Woodworking magazine and perhaps one of Bruce Hoadley’s books. I think a lot depends on the cellular structure of the particular species. Most ‘durable’ woods typically used in boatbuilding have inherent qualities that prevent liquids from penetrating. That’s why they last longer. Some of those tropical hardwoods, like Lignum Vitea, will last forever with no treatment, but weigh a ton! Ash is used for rails because of it’s strength to weight ratio, resistance to shock, moderate durability and cost. Again, it seems that you would need extreme pressure (positive or negative) To achieve even a micron in additional penetration over traditional application of Watco. Excessive number of ‘coats’ will result in a film, so why not just varnish? Aside to Arkay: I can’t blame much on art school 'cuz I can’t really 'member it too good.

And this is why
I titled the original post “Mad SCience”.

It was just an exercise on my part regarding an idea I had for better treating of my ash gunwales. I still have a 50/50 chance of attempting it sometime this winter.

As for the poster who was concerned with the added weight of an oil finish, if I was overly concerned with weight I would loose 100#! I am sure Arkay is right about the slight added weight. The alternative is rotted wood, or aluminum or vinyl gunwales.

It is hard to argue with NT’s treatment of the rails on his Curtis - at the price of multiple passes with ever finer sandpaper and hand-rubbed coats of oil. I am a bit more lazy.


PS: If I didn’t have ash brightwork on my Magic, what would I do to keep busy?

Perhaps not crazy but inventive. Why not just let it soak for a week in the pipe?

Vacuum first
Wow, I stand corrected. But it all makes sense. Vacuum, pressure, vacuum. Which reminds me, my wife is pressuring me to vacuum the den.