Complex sentence: The development of modern aerofoil sections for gliders required very accurate reproduction with smooth surfaces, using tolerances that were extremely difficult to achieve using conventional wood or metal construction, but could be achieved by using composite materials laid up in a mould.
Split into simpler sentences: The development of modern aerofoil sections for gliders required very accurate reproduction with smooth surfaces. The tolerances required were extremely difficult to achieve using conventional wood or metal construction, but could be achieved by using composite materials laid up in a mould.