and I know the first job one must do is to sand and scuff the existing paintwork in order to help the new paint/primer adhere to the surface

Actually the first job should often be to clean the piece if the goal is to provide the best base for uniform adhesion of the subsequent paint coats.

While sanding or other abrasion can remove dirt and surface contamination it's not a reliable way of doing so in certain situations. If the piece has been waxed, or is oily/greasy from use, sometimes you just end up spreading a thin layer of the contaminant over the whole surface as you scuff it. So clean first for safety.

Plain soapy water (at least warm preferably) is enough to do a basic cleaning; you can mix in some alcohol to increase its power if you don't mind the odour. But if you know or suspect there's wax on a piece wiping with white spirit (US: mineral spirits) may be the thing to rely on. Finish maker General Finishes recommend scrubbing with a 50:50 mix of water and denatured alcohol to clean surfaces prior to refinishing or overcoating. It's also very much worth trying some old-school things including sugar soap, but the cheaper and much more widely available washing soda can have the same cleaning power from what I've seen firsthand using both to clean greasy kitchen cabinets. Additionally, both sugar soap and washing soda may dull the finish you're cleaning; more on this below.

And now to sanding, or at least abrading.

• First thing to keep in mind is you're not in any way trying to sand off the previous finish. The very opposite in fact.
• Don't only think sanding.

While you can cover a multitude of sins with paint, including various stains and sand-throughs where bare wood has been exposed, to successfully recoat an existing finish¹ all you should be aiming for is to scuff it up just enough to 'key' its surface, to give the next finish something to grip (i.e. form a mechanical bond). In addition to only needing to sand a little, relatively fine abrasives are all that it is necessary to use. And although you do seek to make the surface uniformly matt you don't need to go crazy; small missed spots and remaining shine in tight recesses aren't a big deal, as long as they're clean.

• The key to adhesion of new finishes to old is clean and matt, or as sometimes expressed clean and dull.

As Bob Flexner put it in one article:
"An old finished or painted surface should be clean and dull for successful recoating. If there is grease or dirt on the surface, or if the surface is glossy, a fresh coat of paint or finish might not bond well. But if an old surface is clean and dull, any new paint or finish should bond fairly well." My emphasis².

And furthermore, you don't need to just sand, i.e. we're not just talking sandpaper. By sandpaper here I mean all the various sanding products (paper, cloth, films and screens), commercial sanding blocks/sponges and the various sanding pads made for power sanders of various styles.

In short, be open to using the two other chief abrasive types: non-woven nylon abrasives (e.g. Scotch-Brite, but don't limit yourself to just that) and steel wool. Steel wool may be old school, but it's a superb conformable abrasive (I would argue personally that it's still the best conformable abrasive³ ) that still has a valuable contribution to make in finishing and refinishing.

For turned legs especially nylon pads or steel wool are a really good alternative to sanding, even power sanding. For abrading the classic in-and-out profiles of spindle turnings, one of the things demonstrated poorly by SurfPrep in one or more videos, I'd argue you'll do a better and faster job abrading by hand. The reason is you can wrap the entire outside of the spindle with the abrasive, grip it with your hand and then rotate — effectively you abrade the entire circumference in one go. Since, as already covered, you're only looking to lightly scuff the surface this can be the work of moments per section; a whole leg might take less than a minute.

Now last but not least....

• In some cases NO sanding or scuffing is required.

As briefly touched on above and as mentioned in various refinishing guides a cleaning process may leave the surface dull. If it's dull enough you may not need to abrade at all.

Bob Flexner has covered this in various articles and in his books, mentioning that both ammonia and TSP (tri-sodium phosphate) in water can dull finishes. So cleaning solutions of sugar soap, washing soda, ammonia and TSP⁴ may sometimes save you the entire scuffing step.

¹ Be it shellac, lacquer, varnish or paint, doesn't really matter.

² This should be borne in mind by anyone who holds to the view that you can't put new poly over old, or can't apply poly on top of other finishes.

³ If it's decent quality and not overly greasy. If you find a brand that holds up to use fine but is a little oilier than you'd like it's not difficult to degrease by soaking in solvent, and the same solvent can be used again and again for this task.

⁴ Note that these are all basic or alkaline.