In general Softwood Timber is best Varnished or Waxed and Hardwood Timber is best Oiled or Waxed, their are exceptions to this depending on what colour and finish you are looking to achieve.
Don’t be put off using an Oil, because modern finishing Oil is easier to apply than Varnish, dries just as quick as Spirit based Varnish and looks better with age.
The process we use to finish Stair parts is as follows
For finishing Redwood Pine, Douglas Fir and Yellow Pine seal the Wood with a coat of Sanding sealer, which will fill the pours of the Wood, when dry, sand the Wood with 150 grit Glasspaper or an equivalent sponge sanding block, you should now have quite a smooth surface, even better if you use a vacuum cleaner to suck the sanding dust out of the Wood.
Do a second coat of sanding sealer and when dry sand with 240 grit Glass paper or equivalent then vacuum again.
If you intend Waxing the Pine, you can now apply a coat of paste Wax which after drying for a few minutes can be buffed up with a soft cloth or to make life easier you could purchase a wax buffing brush that fits to an electric Drill which is ideal for Spindles.
For an even better finish the Sanding before the Wax can be done with Wet and Dry paper (any grit from 400 to 1000) used dry but the Sanding Sealer will clog the paper pretty quick so expect to use a few sheets.
For a Varnish finish, after the last coat of Sealer rub down as before and then apply a coat of clear or coloured Acrylic Varnish with a Paint pad.
Acrylic Varnish does not flow out like spirit based Varnishes so you need to keep using the Paint pad to remove too much build up of Varnish in any tight corners but in warm conditions Acrylic can start to dry as you are using it so stop trying to spread it if this happens, you can always sand out any imperfections before the final coat.
We use Paint pads because they give a better finish than a Brush when using the Acrylic varnish.
For finishing Hardwood ( Douglas Fir can be finished this way as well) apply a coat of Danish Oil using a Brush and let dry (1 to 6hrs depending on temperature).
Sand with 150 grit Glass paper and vacuum the dust.
Apply the second Coat with a Brush but use very thinly this time, leave until the next Day and flat down again ,this time using 240 grit Glass paper.
Apply the third Coat with the Brush but this time it should be no more that a light flick over.
Leave until the next Day and then flat down with either fine Wet and Dry Paper or 1000 grit Webrax which is a modern equivalant of Steel Wool and then Vacuum.
At this point you can either apply a fourth Coat of Oil by using a soft cloth so that it is just a light wipe over or apply a Coat of Wax instead.
If you go with the Oil, you can always add a Coat in the Future and if you Wax you can add another Coat of Wax in the Future but you can never Oil on top of the Wax unless the Wax is removed first with a Wax remover.
Always try your finish on scrap pieces of Wood first to ensure you like the result.
Oiled Rags should be left outside to Dry before being disposed of as in rare circumstances bundled Rags
Soaked with Oil generates heat and can catch fire.