Balsa sheet is the lightest timber used in modelling. It cuts with one pass of a sharp blade, sands down in moments and, for its weight, holds a shape remarkably well, which is why it becomes sheeting, ribs, infill and whole airframes for flying models.
Balsa sheet is graded by density, and that figure decides how hard and how stiff the board is. Everything else about a board follows from it: how deeply it dents, how cleanly it cuts and how much load it will carry before it folds.
A lower density means a lighter, softer board for sheeting and infill; a higher one gives stiffer stock for structural parts. The individual grades are listed as special up to 110 g/dm3, special up to 130 g/dm3 and standard over 131 g/dm3, which correspond to roughly 110, 130 and over 131 kg per cubic metre. Choose by grain direction as well: straight-grained board resists bending, cross-grained board curves more willingly around a former.
