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The myth of BLO

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Forum topic by Clint Searl posted 343 days ago 2539 views 0 times favorited 59 replies Add to Favorites Watch
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Clint Searl

1419 posts in 986 days


343 days ago

One of the more enduring finishing myths is that BLO “soaks” into wood. I’ve been more than a little skeptical about that idea, so I conducted an experiment to see if and just how much BLO and raw tung oil penetrate different woods. I puddled each oil on samples of white ash, red oak, cherry, and hard maple. After a half hour, I wiped the oils off; and after another hour, re-applied the oils. After another hour I sliced the samples to reveal the edge profile that would show the oils’ effects. The open grained oak showed the most penetration at about one millimeter, followed by cherry at around a half millimeter The white ash showed little if any penetration; and the hard maple showed no penetration at all. Between the two oils, the tung oil seemed be absorbed more, which may be accounted for by the difference in molecular weights: 900 for BLO and 250 for tung oil. Because both oils are drying oils, exposure to oxygen begins the polymerization process that impedes the penetration of subsequent coats, which only serve to microscopically thicken the layer of dried oil on top of the wood.

My conclusion is that aside from any aesthetic consideration, there’s no functional reason to us BLO or tung oil under a resin finish.

-- Clint Searl.............We deserve what we tolerate


59 replies so far

View bluekingfisher's profile

bluekingfisher

1008 posts in 1605 days


#1 posted 343 days ago

Interesting!

-- No one plans to fail, they just, just fail to plan

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Grandpa

3073 posts in 1300 days


#2 posted 343 days ago

I agree with you. It will stack up on the surface and protect it some. It is cheap, easy to apply and easily repaired.

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oldretiredjim

180 posts in 1010 days


#3 posted 343 days ago

Thanks for the useful info.

View Don W's profile

Don W

14828 posts in 1193 days


#4 posted 343 days ago

I alway though BLO under another finish was for aesthetic consideration.

-- Master hand plane hoarder. - http://timetestedtools.com

View Dallas's profile

Dallas

2863 posts in 1112 days


#5 posted 343 days ago

In your statistical samples were all woods sanded to the same smoothness?
Also, remember that each of those woods, has a different grain profile.
Was the oil rubbed in? That would make a difference, as would temperature and humidity.
Were these tests done with true tung oil and unadulterated BLO? I have some tung oil that is about 20 years old and it is about as thick as honey. It makes a beautiful finish, but I doubt it soaks in at all.

I have never actually thought about it before, I have never worried about either one being a penetrating oil. I use this stuff for a finish, not for a preservative.

Thanks for the information though, maybe someone can do a more in depth test… Ask the gummint for a grant!

-- Improvise.... Adapt...... Overcome!

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Fred Hargis

1714 posts in 1118 days


#6 posted 343 days ago

Yep, I use for color/grain enhancement. Although if using an oil based finish like varnish, that work seems to be taken care of by the varnish.

-- Our village hasn't lost it's idiot, we sent 'em to Washington.

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ChuckV

2398 posts in 2152 days


#7 posted 343 days ago

Craig Stevens got quite different results on white pine:
http://www.woodworkersresource.com/content/shouldyouheatyourlinseedoilbeforeapplyingit/

In this article, he is testing for greater penetration using heated BLO. He finds no difference.

-- “That it will never come again / Is what makes life so sweet. ” ― Emily Dickinson

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PurpLev

8476 posts in 2274 days


#8 posted 343 days ago

was the wood sanded prior to applying oil?

I agree with the assessment that any penetrating oil finish has no benefit under film finish other than for coloring/appearance changes.

these oils are great finish for items that you want to preserve the ‘wood feel’ but still provide moisture barrier to them irregardless of how ‘deep’ they penetrate. the concept of ‘penetrating’ is to differ these from film finishes which produce a hard finish layer on top of the wood, but that does not mean that these penetrating oils penetrate the wood all the way to the core.

-- ㊍ When in doubt - There is no doubt - Go the safer route.

View CharlesNeil's profile

CharlesNeil

1127 posts in 2496 days


#9 posted 343 days ago

Clint , I agree, I have found no sound reason for the use of either in modern day finishes, we have far better oils and products that do as good a job as the BLO and pure Tung oils. I realize alot of folks use and like them for grain pop and enhancement, but again there are far better drying and more durable products. Just my personal opinion

View Jimbo4's profile

Jimbo4

1129 posts in 1388 days


#10 posted 343 days ago

What if both were cut 50% with a carrying agent, would “soak” rate be any different?

-- BELT SANDER: Used for making rectangular gouges in wood.

View Rick M.'s profile

Rick M.

3861 posts in 1005 days


#11 posted 343 days ago

Oil applied to face grain doesn’t saturate the piece, I expect that is common knowledge. You are demonstrating a property of the wood, not of the oil. Submerge the end grain and it will wick all the way through.

-- |Statistics show that 100% of people bitten by a snake were close to it.|

View rum's profile

rum

148 posts in 1211 days


#12 posted 343 days ago

So a slightly related “experiment” and some other notes.

I turned a bunch of wooden rings from oak about 2.5” across with a 1” hole in the middle (intended to be used as wooden eyes with a rope spliced around them, although the project took a different turn when the fellow I was doing them for got some much larger rope so .. any use for 150 wooden eyes?)” Anyway I soaked them all overnight in a bucket of hardware store BLO mixed with about 20% turpentine. The walls on these are between 1/2” thick and the rim and maybe 3/8” at the center and I can say that the BLO mix soaked all the way through. Granted there was a fair bit of exposed end grain and these were oak (white but still) so these were easy compared to other structures/woods but generally if you really want the BLO to soak in much you have to cut it and soak it a fair bit longer. Some of the rings I got in a hurry on and the ones that were in the mix only 2 hours don’t have as good of penetration as the ones left overnight.

Now this is not actually desirable.for a lot of uses. BLO is oxygen curing so in this case the outside “skin” actually prevents the oil in the core from curing for a looong time (up to years if you get it in there deep enough). In this case it has even kept the turps from evaporating entirely (and that’s pretty volatile). The advantage of this is that if the piece is scuffed or damaged the oil will re-form a protective coat. This is nice for things like garden tools or outside “ships ware” (like the eyes) where you don’t want to be dealing with a film finish that wears/flakes/chips off. For inside furniture you just end up with something that still smells of turps/raw oil and seeps oil out the end grain 6 months/1 year/?maybe longer depending on the wood? later.

This is not to say you can’t make a great film finish from BLO or tung, you can – but optimizing for penetration isn’t the goal in that case. What you want there is a lot of very very thin layers that have been allowed to thoroughly polymerize between coats (heat also accelerates the polymerization so if you put the piece under a heat lamp you can reduce the time between coats somewhat). If you put it on to thick or don’t let each coat fully cure before the next you’re left with a horrid gummy mess you’ll have to scrape/mineral spirit wash off.

I do use BLO, Tung and other drying oils for some types of projects, but I think its worth remembering there is no “one true” finish and you need to think about what the project will be used for and use an appropriate finish and finishing process based on that.

View crank49's profile

crank49

3366 posts in 1596 days


#13 posted 342 days ago

I was taught that BLO is used as an additive to improve the application properties of other finishes.
And that’s how I use it. It will turn a sticky draggy hard to apply finish into a smooth gliding wonder.

Never considered it to be a finish unto itself.

-- Michael :-{| “If you tell a big enough lie and tell it frequently enough, it will be believed.” ― A H

View unbob's profile

unbob

379 posts in 528 days


#14 posted 342 days ago

Try mixing BLO 50-50 with mineral sprits, that is, if you want more penetration. Here is an example where I wanted to darken up hard maple on this leg vise. Perhaps a little darker then I needed it to be, after 3 applications of it.

View Charlie's profile

Charlie

1008 posts in 911 days


#15 posted 342 days ago

I use BLO and Tung Oil (the real stuff) because I like it. I like the way it feels and the way it smells. For my aromatic cedar flutes, I often use Tung oil cut 50/50 with citrus solvent (again… the real stuff) and progressively work to a final coat of just Tung oil. The flute won’t last forever though it will last longer with care from the flautist. I don’t like what a harder finish does to the instrument. This is all personal preference.

I also like shellac, and lacquer. I’ve even been known to use poly (gasp!).
I also like cassein paints (milk paint).

I’m not a finish guru. I don’t know all the chemistry behind all of the available finishes. There are guys like Charles Neal who can give expert and knowledgeable advice on finishing questions.

I use lots of different things because I create lots of different things. I was an artist long before I was a woodworker (hence the recent easel build) and as an artist I like the variety. And I like to use tools suited to the job, but I’m not afraid to go off the beaten path and do something considered “different”.

Finishes are like the icing on the cake. They should be lovingly applied and some things are just for decoration.

Let’s face it. If every cake had chocolate frosting, cake would be pretty boring.

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