My question is about sharpening edge tools such as plane irons or chisels.
How do I establish, when looking at a tool, what grit to start the sharpening process on? It's clearly always possible to start on a very coarse grit and work my way up to fine grits and stropping, but in some cases some of that work at the coarser end is unnecessary. How do I figure out when it's OK (and a time saver) to start at, say, 400 grit or 1000 grit?
In order to do this without gaining the experience to be able to assess what grit would be a good start, you would have to (I think this is why people are saying you really have to judge it.):
To gain experience you really just need to do more sharpening. If you are concerned about removing too much material too quickly then start with a fine grit and see if that polishes some parts and leaves scratches or chips untouched. Keep using more coarse grit until the abrasive produces the same size scratches as you're trying to remove. Then start working your way through finer grits to remove the scratches from the previous grit.
As you improve, you'll naturally be able to look at the scratches or chips and know that it looks like you've just finished with 240 or 400 or 80 (gah!) and that's where you should start.
How do I establish, when looking at a tool, what grit to start the sharpening process on?
Really you don't need to think about it.
In practice you generally don't need to assess your edge, you can just sharpen the tool "as normal" because it will have blunted approximately equally each time1. This is pretty much the norm for working woodworkers and has been standard workshop practice for maybe the last couple of centuries, so no need to buck the trend :-)
One thing that can help a lot with this is not waiting for your tools to get blunt in the first place.
Little and often
This approach doesn't get stressed enough in sharpening guides, especially today and in online guides. It's common among carvers and some whittlers to keep their tools razor-sharp (literally) by regular 'top-up sharpening', generally by stropping.
Stropping is great for top-up sharpening because it's very fast and effective IF you haven't left it too long, 20-30 seconds to get back to shaving-sharp from a little blunt. So there is a lot to be said for adopting the same policy with plane irons and bench chisels also.
You can of course do the job on stones/plates if you prefer not to strop, but it's a little more difficult to do it as quickly! Still aim to be back at work in under three minutes. Worth bearing in mind also is that it's not always necessary to use your finest stones just because they're there2.
You might find some of these previous Answers useful to read over, in part to keep sharpening distinct from honing in your head (both sharpening tasks but usefully separated sometimes):
Sharpening grits -- naming and selection
What criteria would want me to bevel my chisel in a certain way
How does one aggressively sharpen chisels and plane irons when damaged?
1 This obviously assumes there hasn't been any edge damage, which might happen from a tool being dropped or from contact with a particularly hard knot, where minor folding or chips can occur. In this case you would need to sharpen more aggressively, but this is of course rare in day-to-day woodworking for most of us.
2 We should remember that it was common in the past for there to be a couple of oilstones (sometimes only one!) and maybe a strop to be the sum total of each woodworker's sharpening equipment. The problem these days is that so many of us leisure woodworkers are over-supplied with stones and plates to pick from, especially at the stupidly high end, when all most of us actually need for general honing duties is one combination stone (or individual medium and fine stones) and either a strop or a very fine stone when you need an edge to be more refined, very useful on chisels but with plane irons very much not always necessary.
On a plane iron you can work with an edge straight from 250 grit paper as Paul Sellers has commented on in one or two of his videos, which closely equates with some of the edges achieved in the past..... and we have only to look at old furniture made in workshops from these eras to see what those edges could do, an excellent practical illustration of what is good enough.
The basic idea is that you want the grit to make scratches that are finer than the defects you are trying to remove (otherwise the grit will make things worse rather than better), but not a lot finer (otherwise it will take too long).
The only caveat here is that when you are trying to reestablish a main bevel, the relevent "defect" is the layer of metal you are trying to remove (which can be 0.5 or 1mm ).
If just the edge needs sharpening, you may be able to start with a relatively fine grain and just work on the micro bevel at the actual edge.
If you need to establish the main bevel, you may need to go with something more aggressive.
If you need to actually repair the edge, or do the initial flattening of the back of a chisel or plane iron, starting at the coarse end of the scale will speed that up.
Others have covered the looking well, but I find touching is a helpful addition to develop intuition. For example, when sharpening knives I know by running my thumb over the edge sideways if it is 'sharp enough'. A sharp edge will vibrate from your fingerprints - a knife for example will also 'sing' if this is done quickly when sharp. A blunt edge feels slippery by comparison. As far as I can tell the friction increases with sharpness.
