The $100 Makerspace Tuba

Damp rags are more likely to make it worse. Wait until compmetely dry and use a sanding sponge, rather.

What pieces are these? What interior shapes are you working with and how large are they. As @MrDibble suggests sanding the interior and then maybe a coat of something more putty-like that can be smoothed might be a better approach. Maybe light weight auto body filler.

The papier-mache pieces are the curved sections of tube. Each one is approximately half of a torus (except slightly tapered). The smallest one is about an inch in diameter, while the largest is about five inches in diameter.

So you’re making half tubes and putting the together clam-shell style. I haven’t used paper mache much, hardly at all since making a big animal shaped pinata in the Cub Scouts mainly using crepe paper streamers. I have seen in the past and recently on line paper mache powders which may be easily smoothed but don’t really know.

Sanding the interior sounds like the best approach based on my minimal experience. Obviously you want to start as smooth as possible but you probably need to sand to finish. Not sure if it makes a difference but you can bind your paper with white glue which may make it more shapeable as it dries.

Also wondering if you considered rotary valves instead of pistons. I don’t know how actual rotary valves are configured on brass instruments, but perhaps with printed parts not that difficult to achieve.

No, whole tubes. Cut the torus the other way. Like a macaroni noodle (I really should have taken some pictures last time I worked on it). Which, for the smaller ones, makes the interior difficult to access.

And I did consider rotary valves, but decided that piston valves would be simpler. I can’t remember the precise basis for that decision, because that was one of the first decisions I made on the project, six-ish years ago. I was probably influenced by the fact that most (though not all) existing tubas use piston valves. It might also be because rotary valves need more moving parts (you need some sort of linkage connecting the lever you actually press with your finger to the valve, as opposed to just sticking a flat button on top of a piston valve).

How did you (or planning to) make these macaroni elbows? How is the interior shape formed?

I’d suggest some kind of mold but can you remove a mold from these shapes? Maybe make a polystyrene foam mold. Shape it, leave a hollow core so it can be broken apart to take it out. Not sure what to use as a release from paper mache.

OK, I finished the curved tubes on the bottom segment today (aside from allowing time for it to dry), and got some pictures:

The first image was before I did the last layer of newspaper (and hence dry), and the second one is after (and hence still wet). I decided to use comics pages for the outermost layer, for decoration, but the dry pictures probably show the idea better.

I see, you’re doing the tubes in situ, I hadn’t realised that.

Yeah, it’d be too hard to match the shapes exactly, if I did them separately and then installed them.

I did have a new idea, though: What if I made the tubes out of cloth, stuffed them with fiberfill in situ, and then coated the cloth with… something (some sort of resin?) to stiffen it, and then removed the stuffing? All of those steps seem like they’d be much easier than what I’m doing now, and so now I just have to figure out what to use on the cloth. It would mean I’d need to create a pattern for the cloth tubes, but I’m trying to optimize for assembly work, not for design work.

See what you can accomplish with a pool noodle. Specifically a jumbo one. That might not get you the full diameter you need but perhaps a jumbo pool noodle inside a fabric sock with some fiberfill will get you better stiffness

A pool noodle is an excellent idea. A soft wire could be used to hold it’s shape.

The cloth needs to retain its stiffness after the stuffing (whatever it is) is removed.

Use leggings for the cloth. I assume there are some that are seamless.

Bondo (auto body filler) over the cloth will give you stiffness and hardness. You can also spray insulating foam like Great Stuff on it for more strength. It’s a difficult to spray on in a particular shape but you can easily shape it after it dries. Or build up paper mache over the cloth to make it stiff.