At my woodshop, it's typically suggested that I sand up to 220 grit or 320 grit. I'm always amazed at how smooth my surface feels after a good 320-grit sanding. However, when looking online I sometimes see advice to sand up to 600 grit. 600! That must be like glass!
I understand it's a matter of taste vs effort, but at some point there must be too little improvement to take another pass with a higher grit. How do I know what this point is? Does it depend on the finish that I'm going to apply?
I understand it's a matter of taste vs effort
Actually not as much as people tend to believe.
Sanding to 320 grit may itself be too high, and above 400 you can definitely run into problems. Some woods burnish when rubbed with higher-grit papers and this can prevent the proper penetration of finish, which helps ensures a more even colour and the formation of a proper bond between it and the wood. Burnished wood can resist finish, even to the point that it will bead on the surface sometimes and to fix this you need to re-sand at a coarser grit to 'open' the surface of the wood again.
What determines the grit you need to sand to is not so much personal preference but rather whether you are finishing the wood and with what.
Oiled finishes require a more meticulous approach to final smoothing because they don't build a film and after oiling minor surface imperfections are magnified, not hidden.
With a film-building finish (shellac, varnish, lacquer) you get a filling-in of minor surface imperfections, including sanding scratches. This is why you can get away with sanding up to a coarser grit and still achieve a very smooth final surface.
Using a film finish, depending on the species, even 150 can be high enough. But the normal range to finish sand is between 180 and 240.
If you do not intend to apply a finish — which can be used for dense hardwoods, particularly tropical species with a naturally high resin content — then you can sand to very high grits indeed because you're actually polishing the wood itself. Even beyond the 600 you mention, I've seen the results of sanding past P1600 and you do get increasing shine, up to a limit dependent upon the density and resin content of the wood.
Adding to glw some of it depends on the wood. Some woods I've used 220 is as good as you are going to get, anything higher doesn't do anything and sometimes actually makes it look worse. Pine and other softwoods really don't benefit much from higher grits.
Red oak maxes out at about 320 anything higher and your just wasting your time. White oak is debatable, 400 is does a little but isn't really necessary.
Hard Maple I've sanded to 600 grit and you almost get a semigloss finish before you put anything on it. The denser the wood (with a closed grain) the higher the grit can actually make some difference. I suspect ebony would glow with a sanding up to 1000 grit.
For most cases in which you intend to put on a film finish (like shellac, lacquer, or a varnish) it is enough to sand the wood to 220. (I suppose the same would be true if you intend to use an oil finish too). Nothing wrong with 320 - if that's all the store has, it works more or less the same.
Higher grits are really for sanding the film finish after you've built up enough thickness not to sand through it. These higher grits will allow you to create ever more polished finishes.
Adding again to the above answers I'd say that it hugely depends on the type of wood and finish, but also on the method of sanding. Sanding by hand, with a "pad" type orbital sander, or with a disc type random orbital sander with the same grit will give you pretty different finishes.
We use random orbital disc sanders for finishing at our joinery workshop on Sapele, Idigbo and European White Oak and always use 120 grit. This gives an excellent finish when painted with a spray-applied water-based microporous paint or stain for external joinery.
I also find that with certain timbers and in certain conditions, finer grit papers tend to "gum up" - you get dust stuck to the paper which then leaves indentations in the timber rather than continuing to sand it, which can be counter-productive.
