Dr. Science plays with wood finishes - Part 1

scientific method,

n a formal style of study or research in which a problem is identified, pertinent information is assembled, a hypothesis is advanced and tested empirically, and the hypothesis is accepted or rejected. [The Free Dictionary]

Well there you go!

I'm looking to figure out which is the best wood finish to put on my SKS gunstock as a first coat.

I have some requirements:

1) Must sand well. It will be sanded at either 320 grit or 400 grit sandpaper, either wet or dry with the 'wet' part being some of the same liquid used as the sanding sealer coat.  It must not clog up sandpaper.

2) Must be able to dry in one day.  I will not wait a week for it, pretty simple.

3) Must be able to take the higher temps next to the barrel and receiver in the inlet areas.  I can do a two-part finish, as I will not be sanding the inletting save to clear out any rough spots.  The temps of 180 F or more means that shellac is out unless it is part of a higher temperature varnish.  I am still researching this.

4) Must not prohibit the use of other finishes over it.  In other words, it can't have wax in it or be too acid/base to actually make it difficult to get a coat over it.

5) Visually appealing for beech wood.  Grain type, variation, and texture must show through.

6)  Nothing fancy.  No ground coat that is opaque save once the proper varnish with the right refractive index is put on to suddenly pop all the grain coloration through.  This is, in other words, an SKS gunstock, not a violin or viola.

7)  Need not match 'arsenal' look.  The arsenal botched the job.  If I wanted a gunstock with a botched-job look, I wouldn't go through all this.

8)  Not absolutely necessary, but if it can stick to easily available materials or equivalents to what an ordinary Yugoslav soldier would have while in service (meaning on-base or at any nearby town), it would be a big plus.  Nice but not necessary.

9)  Low maintenance, able to take regular use well and have some relatively easy method to fill in abrasions and scratches the finish will pick up over time.  Nice, but not necessary.

10)  Hunting scent friendly.  Nice but not necessary.

 

Notice that 'easy to get', 'cheap' and a few other choice requirements are not, necessarily, part of this.  That would be nice, yes, but I can find some hard-to-get stuff for making varnishes and such.

My initial line-up looks like this:

 

From left to right are: Tung Oil (from The Real Milk Paint Co.), Boiled Linseed Oil (aka BLO) from some big box store on the cheap, Virginia Cedar oil (from Something Cedar), Citrus Solvent (also at TRMPC), and Krud Kutter brush wash.  Not seen are four stainless steel mixing cups, cheap acid brushes bought bulk from so many places its not funny, Slip 2000 725 Cleaner-Degreaser for cleaning out the cups (from Midway USA), and KG-3 (also from Midway) for the final cleaning of the cups after a day of using them.

The first thing I want to do is find out how each of the oils work separately for application at various concentrations of oil to solvent.  Solvent is used to dilute the oil, break up the longer chains and decrease drying time.  Solvent also lowers the oil viscosity so that it is easier to apply and spreads thinner.

For each of the oils I utilized ratios so that there was an amount of oil to an amount of solvent.  This would be done at levels of Straight (no solvent), 3:1 oil to solvent, 2:1 and 1:1.  Any sanding sealer layer must go on thin, dry quickly thusly being able to be sanded readily to fill in pores or otherwise level out the wood grain.  Often a second sealing layer is used to do a final smoothing out and filling of the wood.  To test this out I applied each in the concentrations given, and allowed a two hour to dry blot time to the 2:1 and 1:1 batches and then an overnight for the Straight and 3:1 applications.

Results:

The same with the board reversed for lighting and flipped around for display to remove lighting as a factor:

And the back side for comparison:

For the board there is a top to bottom line-up of Tung, BLO and Cedar oil, and across are Straight, 3:1, 2:1 and 1:1.

Notes:

- The wood is purchased in board lengths from Home Depot.  Their supplier marks this as 'white wood' and is from Sweden.  It is inexpensive at 1x6 dimensions.  It does not have any odor of pine to it.  It is relatively high density and relatively easily worked along the grain in a router.  It chips when routed across the end grain.  Now you know as much as I do about it.

- Tung oil goes on smoothly, loses viscosity evenly so that each addition of solvent makes it flow faster, and it pops out the grain of the wood, visually.

- BLO rapidly goes from a thick oil to one so watery that it is hard to contain at 2:1 and 1:1 concentrations.  It puts a yellow cast to the wood and does not pop out wood grain, although it did not have as much to work with as the Tung oil.

- Cedar oil is relatively low viscosity compared to the other two oils to start with, comparable to 2:1 concentrations of BLO or 1:1 of Tung oil.  While solvent increases volume it only substantially lowers viscosity at 2:1 and 1:1 where careful application is necessary to keep it in the test areas.  Even at Straight it tended to flow to the left of the board.  It has an earthy aromatic quality of cedar that is not as biting as western cedars.  Cedar oil does some popping of the grain, but due to low viscosity it is unknown if the BLO just above it was mixing with the cedar oil.

- Comparatively BLO flows faster with additional solvent as compared to Tung oil and introduces a moderately more yellow cast to the wood.

- Cedar oil has an initially high pop factor for the grain, visually, but that goes down as it dries.

- At 3:1 concentrations BLO was gelling up overnight and came off in a spotty fashion.  This was not the case with the Straight application.

- All are well suited for a 2:1 or 1:1 concentration with 2 hour blot time.

- Tung oil was amenable to an overnight blot down for the Straight and 3:1 concentrations.

- Cedar oil was dry along non-border regions with BLO at Straight and 3:1, and blottable at the margin area.

 

Conclusions

The reported ease of use of BLO reported anecdotally is one that is indicated by this simple test: at 2:1 and 1: concentrations with a citrus turpene, BLO flows very easily.

Tung oil is comparable to BLO at 2:1 and 1:1 concentrations and superior at 3:1.  At straight it is similar to BLO.

BLO at 3:1 concentrations is a tricky business as it can, apparently, be wiped down before a longer over-night sitting.  As it starts to thick-dry, it is hard to work with, taking some of the blot material which then needs to be rubbed off, and that takes off the gelling BLO.  A 3 hour blot time would, apparently, be safe with BLO at 3:1 concentrations.

Cedar oil leaves no yellow cast to it on the wood, and is superior to both BLO and Tung oil for leaving as much of the wood color coming through it at all concentrations.  As an aromatic oil it dries quickly, save at Straight concentrations overnight where some was left in non-border regions with BLO, but those cleaned up easily with a dry blot disposable wipe.  Compared to Tung oil and BLO the finish left by cedar oil is relatively thin, indicating that it would need a post-sanding coat if used as a sanding coat.

The surprisingly rapid miscibility of cedar oil and BLO needs to be investigated.

Next it will be mixed with both, and with solvent, and all sections given a 2 hour blot time as the aromatic nature of cedar oil indicates it speeds drying time.

Another oil will also be tested in this on its own board both straight and with cedar oil and solvent.  It is not a regular modern gunstock finishing oil.