I've been asked to prepare a 3D model for 3D printing in sandstone. I've been told that it needs to be 3" tall and the walls have to be at least 2 mm thick. It's an absolute pain in the neck having to make sure everything is the right thickness. So, I was wondering, can the whole model just be printed as a solid object, with 100% infill, and does that work for sandstone?
Yes, you can just print it solid. However, it might be significantly more expensive to print your object entirely solid. For instance, Shapeways charges \$0.75 per cm³ of material for their full-color sandstone. A solid cube of 5x5x5 cm would cost \$96 to print, whereas it would only cost around \$6 if you printed it hollow with 2mm walls.
To add to the selected answer, again, yes you can, however making all of the walls AT LEAST 2mm thick shouldn't be too hard, depending on your model and modelling app.
If your modelling app doesn't have a good shell function (or if it's too finicky on your model) you could easily define SOME negative space within the larger volumes of your model in order to save costs without having to actually make perfectly consistent wall thicknesses. Just make sure you leave a hole through which the excess material can escape from the interior void.
Shapeways has a good set of guidelines for their full color sandstone prints, which work about the same as any: http://www.shapeways.com/materials/full-color-sandstone However, you'll want to double check exact measurements with your printing service.