Est. 1998 — 27 years of woodworking knowledge
Thinning for Penetration
Don Stephan
In the last week I've seen several suggestions here or elsewhere to thin a finish so that it penetrates deeper into the wood. Anyone done any experiments. My expectation is it would depend on several factors - size of the finish molecules, polarity of the cells and molecules, . . ., and that it would sometimes help and sometimes not make a difference.
Re: Thinning for Penetration
Clint Searl
I did some simple ad hoc experimenting with BLO and pure tung oil on cherry, maple, and oak. On even the more open-pored oak, penetration was only about 1mm. More coats and/or reduction would have no effect as polymerization of the oils at the surface of the wood would impede any greater penetration. Moreover, even if there were deeper penetration, it would have no significance in either performance or appearance.
My conclusion was that a film finish is superior to a drying oil and could be manipulated to produce any desired combination of appearance and durability.
Re: Thinning for Penetration
Jesper, Denmark
The molecule size will not be a factor.Molecules are extremely small in relation to the voids between the wood fibres - even the large molecules.
Viscosity and surface tension of the liquid finish are much more likely to be the governing influence.
Re: Thinning for Penetration
Josko Catipovic
I use WEST epoxy thinned with acetone to stabilize rotting boat transoms and other wooden parts. Works extremely well, and I'm frequently amazed when I see an inch of penetration in softening wood. If wood is thoroughly dry it will penetrate amazingly far, and form a sort of rot-impervious matrix very like the surrounding wood.
Re: Thinning for Penetration
John C Lucas
I have had System 3 mirror coat epoxy penetrate all the way through red oak end grain. .havent tried to.see if it will.penetrate side grain woods.
Re: Thinning for Penetration
JL
Experiment.
There are a lot of variables.
I typically thin Minwax gloss polyurethane - because I feel I get better penetration when it is thinned.
IMO there it is too viscous out of the can to penetrate deeply. I have cut pieces where I applied thinned and normal polyurethane and looked at them with a dissecting microscope. There is deeper penetration with the thinned gloss polyurethane.
But every finish is different. Not all of them have the same viscosity. Not all the finishes (or wood) are at the same temperature.
Re: Thinning for Penetration
Don Stephan
Red oak cells remain open, which is why wine and whiskey barrels are never made using red oak. Years ago I saw in an episode of Roy Underhill's show he put one end of a red oak dowel in a bowl of water, blew on the other end, and bubbles appeared.
Re: Thinning for Penetration
Clint Searl
What is achieved by "better penetration"?
Re: Thinning for Penetration
JL
Better penetration means you can't strip it easily 
and if you mess up, you have to sand a lot more.
So - better bonding of the finish with the wood.