One side or both sides of the panel 20′″ may be covered with {HEAD ~ a layer}, such as {TAIL ~ an outer covering layer} 
One side or both sides of the panel 20′″ may be covered with { HEAD ~ a layer } , such as { TAIL ~ an outer covering layer }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL