It differs.
Some seems to be lacquered (f.e. Beomaster 1200 and early Beograms), others are thin melamine (70's Beovox speakers) or an even thinner adhesive surface (80's Beovox speakers), yet others have cast white plastic panels (Beomaster 1900/2400 and some 70's Beograms).
None are easy to restore or replicate.
Martin