The below line works correctly:
(custom-set-faces
'(default ((t (:family "Liberation Mono")))))
but when I try to retrieve the font-face from a variable, it fails
(setq font-family-default
"Liberation Mono")
(custom-set-faces
'(default ((t (:family font-family-default)))))
Is there not a way to refer to a font-face through a variable/symbol?