How to add syntax highlighting inside comments for an existing major mode

[Julien Barnier]

Hi all,

I am trying to customize some syntax highlighting for an existing
major mode. What I would like to do is to display different type of
comments with different faces.

In this mode, comments are lines that begins with one or several
«#». What I would like to display differently those which begin with
«#####» than those which begin with «####», etc.

I tried the 'font-lock-add-keywords' way, but it doesn't seem to work
inside comments, with something like that :

(font-lock-add-keywords 'ess-mode
  '(("^##### .*$" . my-beautiful-face)))

Do you have any idea for a proper way to do such a thing ?

Thanks in advance for any help.

-- 
Julien