As I was polishing the 3D printed pattern for the flywheel you show in your first photo, I was generally trying the sponge on everything, including the curved spokes, and the flat surfaces.
The wide flat rim was more difficult to sand out with the sponge, but it was much more flat to start with.
I think the flat surface was flexing away from the sponge a bit.
The spokes seemed more rigid, and had taller lines, and those came off more easily with the sanding sponge.
With a casting as good as the first photo, I would just remove the flash at the parting line, face the rim top and sides, and then very lightly go over the remaining surfaces with a fine sponge, just a very light touch thing.
Some folks will buff or paint everything to a mirror finish, but if the surface is not machined, I prefer it to have a sand-cast texture to it.
The second photo casting came out with a perfect surface finish, so I am not sure what slight of hand you used on that, but as you mention, it is all about the sand.
With the bound sand I use, it is going to mirror things the thickness of a hair, and smaller.
The sprayed on ceramic mold coat helps smooth things out.
I am wondering if enough mold coat could actually fill in the grooves in the sand, so that the 3D printed part did not have to have much smoothing work done to it.
The ceramic sponge method appears to be very promising, and while it will not be a substitute for a full surface fill and sand finish, I do think it will be an excellent surface prep prior to a fill/sand that will greatly reduce the work required as compared to just a fill and sand.
I think 75% of the heavy work can be done by the sanding sponge, and I think many surfaces can be completed after sanding with the sponge just by using a few coats of shellac, instead of putty/filler.
I hope to start 3D printing some Monitor patterns in January, and then I can give a better report on the sanding sponge/shellac method.
Many of the parts on a Monitor are convex, and that lends itself to a sanding sponge.
I am also going to research how I can improve the surface finish by tweeking the Prusa slicer program.
I think the sanding sponge may work ok with smoothing a filled surface.
I tried it on the auto body putty on the dog, and it seemed to take a light cut without digging or creating flat spots.
And lastly, it occurred to me as I was trying the sanding sponge on JasonB's flywheel pattern, that there was no need to buff off any surface that would be machined, such as the outside and faces of the rim, the faces of the hub, etc.
I may try fogging on shellac, and creating a surface that is semi-rough, and something that would bridge the grooves, sanding lightly, and then applying a final overcoat of shellac.
I will try some things, but I think the sponge is going to save a lot of time; I can already see that from early trials.
For smaller 3D printed patterns, and hard to access recesses in patterns, the sponge will be of no use.
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