I am having problems with an Assembly exporting as an STL file

I have an assembly with parts and surfaces in it it is of a boat for a trophy the sail is the problem it is a surface but when I export it as an.STL file the surfaces do not show up I Can, however, export it as a part by its self as an (.STL file )

the surface is the sail all other parts in the assembly are 3D body's and all ok 

Has anyone come across this  problem   did not wish to make the sail a body as it has an image on it and been used as a laser image inside a glass block has anyone come across this before  

is it posable to mix Surfaces & body in the same assembly The sail does save but does not show up in the final STL file  when you open the .stl file 

I did it as a test first all from one part and it was all ok 

I am using SW2017