How to finish plywood ceiling

[blackberry]

We have plywood ceiling. We like the natural wood grain look, prefer not to use polyurethane. I was going to put danish oil on but people I talked to don't think it's a good idea. Any advice, please?

[Nestor]

Blackberry:

If what you have now is bare plywood, then the worst thing you can do is leave it bare like that. Any coating you put on is better than leaving the wood bare because bare wood is porous, and stains easily. Also, airborne dirt like cigarette smoke or cooking oil is going to accumulate in the porous surface of the wood and be much more difficult to remove than from a smooth finished surface.

Danish Oil is a "drying oil" finish which means that it's an oil that dries to a solid when exposed to oxygen. It's typically made out of Tung oil or Linseed oil and often the oil is polymerized, which means that the oil consists of "clumps" of Tung or Linseed oil molecules rather than individual oil molecules. Pre-polymerization at the factory shortens the drying time of the product when it is used. Also, the Danish oil can have dried plant resins (called "copals") dissolved in it to make it dry harder than Tung or Linseed oil alone.

You should be aware that ALL "oil based" finishes, which include paints and clear coats, will yellow if they're kept in a dimly lit environment. What most people aren't aware of is that the yellowing that occurs in oil based finishes is completely reversible. By exposing yellowed oil based paint or varnish to direct or indirect sunlight, the yellow discolouration will completely disappear in a matter of a few weeks, but that yellowing will restart once the oil based finished is moved back to a dimly lit environment. This is why you never see yellowing in exterior oil based paints that are used outdoors. Similarily, you never have yellowing of window sills painted with oil based paint because they're always exposed to sunlight.

So, if your rooms get a lot of natural sunshine through large windows, I don't see a problem using Danish oil on your ceiling, especially a Danish oil that's made from Tung Oil (which yellows less than linseed oil). But, in the rooms that don't get a lot of natural lighting in them, you can expect the oil to yellow in time. (Also, never use a drying oil based finish in a bathroom because the dried oil is a food source for mildew.)

If you'd prefer that your ceiling didn't yellow AT ALL, what you might want to do is apply an acrylic finish, like Minwax's "Polycrylic" water based finish or Varathane's "Diamond" water based finish. These products are typically milky white liquids which dry to a crystal clear film.

Minwax® Polycrylic® Protective Finish | Interior Clear Protective Finishes

Varathane Diamond Interior Water Base Wood Finish | Windsor Plywood


I'm not a woodworker, but I know that shellac comes in various shades including clear, yellow, amber, orange and even red. You could simply paint your ceilings with a light amber shellac. Shellac is a natural resin that dissolves in alcohol, and so alcohol will evaporate from shellac as it dries. So, if you opt for shellac, make sure to provide plenty of ventilation in the room. Shellac won't yellow with age, but will still be transparent enough so that you'll be able to tell that the ceiling is wood.

And, in all cases, if I were doing the work, I'd cut in at the wall/ceiling corner and around ceiling light fixtures with a sash brush and then fill in the ceiling with a 10 inch paint roller sleeve. I'd screw a 5 foot pole into the paint roller frame for easier access, and I'd make sure to cover everything in the room with drop cloths and remove any curtains from the windows before starting to paint the finish on.

Any one of these options would be preferable to leaving the wood bare to get dirty and stained.

PS:
Most people don't know how to use a sash brush properly.
You use it like this:


NOT like this:


That is, the bristles on a sash brush are cut at an angle. That's NOT so the brush can fit in a corner better, but so that all the bristles flare out the same amount when the brush is used properly. That uniform amount of flaring out of the bristles makes it easier to paint a nice sharp line in the corner where a wall meets a ceiling, or where a floor meets a wall as shown below: