Complex sentence: Avetisian's method differs from the method of plein-air painting which was once widespread in Armenian art. For him working from nature was no more than a preliminary stage, and the main portion of the work on the canvas was done in his studio.
Split into simpler sentences: Avetisian's method differs from the method of plein-air painting which was once widespread in Armenian art. For him working form nature is no more than a preliminary stage. The main portion of the work on the canvas was done in his studio.