Should work if you press just Backspace (so as to not remove the border verts) instead of CTRL+Backspace. Or with all the edges selected, switch to border mode, deselect everything. Or select border, invert edge selection.
How to remove interior edges of a selected polygon group while keeping borders intact in 3dsMax? I can do it by a single command in Modo but couldn't find any tools or scripts that does it in 3dsMax.
No worries, sometimes brain just won't cooperate :smile: Btw. apart from CTRL+SHIFT clicking another subobject mode to convert one selection to another, borders excluded, there's also CTRL+clicking (select all the edges/verts/etc. used by the selection) and SHIFT+clicking (select just the border).
Why would it? Select the planar set of faces (for example with By Angle active), CTRL+SHIFT+click the edge subobject icon to select all the edges shared by the faces except for borders, CTRL+Backspace remove them.