|Forum topic by Douglas Bordner||posted 06-18-2007 11:19 AM||3670 views||21 times favorited||21 replies|
06-18-2007 11:19 AM
Reading along, I guess the distilled wisdom I am seeing here can be summarized as follows:
Oil and water don’t mix.
If you use oil of any sort spread rags out to dry before discarding-MAJOR spontaneous combustion hazard
BLO, BLO/MS, Watco etc. make an easy but not tremendously durable finish, described as “in the wood” because the protection (not much) soaks into the wood as opposed to forming a film finish that builds atop the wood. Often LJs will use oil as a first step to “pop the grain”. Since Linseed Oil is well, oil, it’s best to not apply water based finishes over the oil unless it is thoroughly cured and for best results it should be sealed with dewaxed shellac before applying the water-borne finish. Most of these products are polyurethane varnishes.
Others have suggested that, for the sake of durability you could just top coat with Oil-Based varnish, skipping the sealcoat shellac, as oil+oil is okay. You could really skip the BLO entirely, as the oil in the oil based varnish will pop the grain on it’s own. You can also make your own home-brew Danish oil by mixing a third each Mineral spirits, Boiled Linseed oil and a short-oil varnish like Pratt and Lambert #38 (a high proportion of alkyd resin in oil). This allows the flood on, wet sand in, wipe off ease of just BLO with the film build of brushing varnish. Not as durable as Varnish alone, but easier. The deal with oil based finishes is time. Takes 12 hours (generally-maybe longer) to dry before allowing a next coat.
Shellac, good for sealing and so much more
Why waxed at all. Shellac is, to be frank, bug secretions. Bug nests made of largely shellac are found in the trees in tropical localities like Assam and Thailand. They are harvested and refined. The degree of refinement results in the different hues of shellac, the addition of a heating step yields Button Lac which is polymerized shellac which is tougher (also tougher to mix into liquid shellac with denatured alcohol). Denatured means toxic Methyl alcohol added to Ethyl alcohol so unless drinking Sterno is your thing you will take a pass on even thinking about taking a swig of denatured alcohol. But after mixing, all shellac at first has wax in it. It must be mixed, allowed to settle, and the dewaxed shellac poured off (decanted) the top of the wax layer, and filtered. Dewaxed flakes have been processed this way before being re-dried into flakes.
Some folks like the waxed variety for the antique quality it imparts to the work. Pre-mixed waxed shellac is easy to find, but before the popular re-emergence of its use by woodworkers, it was pretty hard to find fresh premixed, waxed shellac. Always check the expiry! And always (always in all finishing situations) prepare a test board using all the steps you intend to use in your project. You don’t wanna be scraping off sludgy shellac on the day before the last shipping date for Keokuk. Trust me on this one.
My favorite shellac is Zinsser Sealcoat. It’s a dewaxed pale blonde 2 pound cut of shellac in the can. It’s thin enough to soak into the wood a little and is not excessively viscous and sticky. You can use it for a washcoat or build it to a higher film coat. It has a long shelf life. It seems to move through the big box stores fast enough that there is fresh finish with a reasonable shelf life left. If that science class feeling is strong you could even add flakes of a different hue to the Sealcoat. It can also be toned with Wizard Tints, TransTints, M.C. Campbell Microtone etc.
The cool thing about shellac: It drys fast. Pad, brush or spray it on fast. Do not tarry. Do not attempt to rebrush it to level the finish. Slap it on. Let it dry. Level it with 320 grit paper or higher for greater sheen. I make jewelry and other small boxes. I tend to finish the pieces before assembly (at least the interiors). I can do the six board surfaces that make the box in a session. By the time the first padded-on coat is on the last board, I can start the next coat on the first board. I can fine sand to a powder, leveling the finish within an hour. It can be rubbed out to varying degrees of sheen within 24 hours. Shellac is reasonably durable for box interiors. Most folks aren’t going to set their glass of neat scotch inside their jewelry box. Shellac doesn’t leave a lingering odor on the interior of a box (or drawer) as oil base products will. If I finish the insides with shellac when the boards are flat, rub them out to a nice flat three coat finish and put a bit of furniture wax on them (mask off any joinery from the shellac and the wax), I can glue up the box, let it dry and cut the box apart. Any glue squeeze out (including big foamy polyurethane glue) just pops off of the insides of the box. Clean interiors — nice joints.
Now the poly thing
When it has to stand up to kitchen abuse, or Uncle Harry who will put his neat scotch glass down on it,
I wipe on thinned Pratt and Lambert #38 Alykd Interior Varnish in the winter, and build many coats. I try and put on as many as I can until my wife becomes annoyed. I rub it out incessantly until my inner German is satisfied. This looks great over cherry, walnut, dark woods.
I have rambled on enough. There are great books out there. Great links on the web.
-- "Bordnerizing" perfectly good lumber for over a decade.