Is it possible to have a persistent canvas background color ?

["Maurizio Ferreira" <ferreira@split.it>]

I tried to assign a permanent background color to a canvas
calling   (send (get-dc) set-background back-color)
during class initialization :

   (define MainForm (make-object frame% "My program" #f ))
   (define MyCanvas (class canvas% (aframe)
                      (inherit get-dc)
                      (override (on-paint (lambda () (do-paint (get-dc)))))
                      (sequence (super-init aframe)
                                (send (get-dc) set-background back-color))))
   (define the-canvas (make-object MyCanvas MainForm))

I wanted to avoid to set a background color and to call clear in the paint
procedure, since (it seems to me) the canvas is automatically cleared
upon entry to the paint procedure, using a (default ? ) white color.

This is my paint code:

   (define (do-paint dc )
     (let*-values (((width height) (send MainForm get-client-size))
                   ((x) (quotient (- width board-size-x) 2 ))
                   ((y) (quotient (- height board-size-y) 2)))
       (set! left-margin x
       (set! top-margin y)
       (send dc set-background back-color)      ; !!!!
       (send dc clear)                                        ; !!!!
       (paint-board dc board)))

I wanted to remove the lines marked with !!!!!

I see that with the two lines above the canvas is cleared two times: one
automatically,  and a second time by my code.
The problem is that if I remove such lines,  sometimes, resizing or
uncovering the frame, the background is painted with the usual dully gray
color.

Any suggestion ?

Maurizio