"2 layers, fill, hollow"

[timid flame]

I need to make a second layer in a hollow and complex structure, but I don't understand how to do it.

I tried filling a giant cube by replacing all the air with a dense block, and then removing the outer part, but it doesn't work that way.

[upbeat remnant]

do you want to make your structure hollow but adjust the thickness of your walls?

like 3 blocks thick instead of only 1 block

[timid flame]

Yes

[timid flame]

now I'm doing this, I filled all the air with a block, and I remove it from the outside so that I can remove the excess from the inside with a mask, but leave a layer.
It's really exhausting.

[upbeat remnant]

you could magic select your structure and use editor mode > main menu bar > select > shrink 3 > and hit backspace

[upbeat remnant]

use the floodfill tool for that

[timid flame]

there is a small problem - my building is already hollow xdd

[upbeat remnant]

make it solid again
use the floodfill tool
or the fill unreachable operation