I don’t know that I qualify as a ‘real builder’ but I do have a pretty complete shop and a couple ideas about sanding:

If you have some spray adhesive around, which you should if you’re running a responsible workshop, then sanding blocks are wildly easy to make, and can be customized for a very wide range of applications. This also assumes an actual shop stock of sandpaper in various grits, but I can’t imagine running a workshop without that. Invest in a stock of sandpaper somewhere where the prices are actually good. You won’t regret it. Some wet-or-dry in your stock will give you good options for sanding metal parts too. You might be surprised at the ‘machining’ operations you can get away with using wet-or-dry mounted on a hard surface

I keep a little rack of sanding blocks that I have made for specific jobs around and if I can’t find a block to do the job, I make a new one in a few seconds from scraps of hardwood, hard plywood, whatever, sometimes customizing the block shape on my tablesaw first.

If I’m sanding something where the ‘hard edges’ of the block might catch and score the material adversely, then I’ll quickly round just the edges of the block… with a sanding block, of course… and then stick on the sandpaper of choice.

for some jobs, it’s particularly important to have a flat, hard ‘reference’ surface for your sandpaper, and for those I make a point of using MDF or melamine covered particle board.

For other jobs, it can actually be bad to have the sanding surface too hard, and here,EPS or blue foam. Sandpaper spraymounted to a small scrap of EPP can make a somewhat flexible block for some jobs.

Basically if you keep around the technology to stick sandpaper to stuff, then your imagination is the only limit for what kind of sanding tools you can create.

Aluminum extrusion, flat or T or L in section can be a great sanding tool.

The other ’trick’ in my book is more of a ‘concept’ than a trick and that is to stay open minded about what moves and what doesn’t. Some small parts are MUCH easier to sand holding the part and dragging it against a fixed sanding surface. I use melamine covered particle board for all my workbench surfaces (for SO many reasons, don’t get me started), and that lets me lightly spraymount down a piece of sandpaper (whole sheet, half sheet, or just a tiny strip, depending on the job) to a very flat hard surface that is truly heavy and unmoving when ever and where ever I want. This is incredibly handy. Usually, if I’ve used the right amount of spraymount, the paper comes back up just fine from this surface. If not, the surface is so hard that I can scrape any residue off with a few quick swipes of a cabinet scraper.

You can achieve this ‘fixed block’ idea by clamping down a sanding block, but that’s fussy by comparison, the clamp is usually in the way, and once you try the deal where a small part of your worksurface magically becomes abrasive, you’ll likely be hooked.

I also keep scraps of aluminum and plastic tubing around for making round sanding tools. Cutting a narrow strip of sandpaper and spiralling it up the tube can often be easier than trying to wrap a rectangular piece around the tube.

Ever get frustrated trying to make small roundbottomed grooves in balsa or foam? Threaded rod is just the ticket… comes in lots of sizes and acts as a very effective rasp in these soft materials. Look for types that have the sharpest ’thread edges’.