OK, I give up. Is skinning cloth the same as fiberglass? Just a new term to me.

Paint. Paint has weight obviously. Plan your paint carefully to keep the weight down. Believe it or not dark colors weigh more than light colors because they carry more pigments. One reason full scale planes are usually painted white base colors. Use a minimum of thin coats to do the job. Surface prep goes a long way toward reducing the need for thick coats.