Correct way to use the start-process function - emacs

Question

I have a small ELisp package that adds an External Tools menu to Emacs. It works on Microsoft Windows but I am having difficulty getting it to work on other operating systems. On Microsoft Windows I use the w32-shell-execute function. On other operating systems I use the start-process function.
My external-tools--exec function is as follows.
(defvar external-tools--exec-count 0)
(defun external-tools--exec (command &rest args)
(if args
(message "(external-tools--exec %s %s) called" command (mapconcat 'identity args " "))
(message "(external-tools--exec %s) called" command)
)
(setq external-tools--exec-count (+ external-tools--exec-count 1))
(cond
((fboundp 'w32-shell-execute)
(if args
(w32-shell-execute "open" command (mapconcat 'identity args " "))
(w32-shell-execute "open" command)
)
)
(t
(let ((external-tools--exec-process-name (format "external-tools--exec-%i" external-tools--exec-count)))
(if args
(apply 'start-process external-tools--exec-process-name nil command args)
(start-process external-tools--exec-process-name nil command)
)
)
)
)
)
This is an example of how I am using it.
(defun external-tools--explore-here ()
"Opens Windows Explorer in the current directory."
(interactive)
(let ((dir (external-tools--get-default-directory)))
(when (fboundp 'w32-shell-execute)
(w32-shell-execute "explore" (format "\"%s\"" dir))
)
(when (and (not (fboundp 'w32-shell-execute)) (executable-find "nautilus"))
(external-tools--exec (executable-find "nautilus") "-w" (format "\"%s\"" dir))
)
)
)
The external-tools--exec function works if args is nil, but it does not work if arguments are specified.
I would appreciate any advice on how to fix the external-tools--exec function.
Edit: I modified the function so that it does not use the convert-standard-filename function as Stefan recommended but the function still does not work. When I use the external-tools--explore-here function on GNU/Linux, I get the following error.
Unable to find the requested file. Please check the spelling and try again.
Unhandled error message: Error when getting information for file '/home/bkey/src/SullivanAndKey.com/SnK/Emacs/Home/.emacs.d/"/home/bkey/src/SullivanAndKey.com/SnK/Emacs/Home/.emacs.d/"': No such file or directory

Answer 1

I figured it out. The bug was not in the external-tools--exec function. It was in the calling function, external-tools--explore-here. I enclosed the directory in quotes, thinking it was necessary to deal with the possibility that there may be spaces in the directory path.
This turned out to be unnecessary.
The new function is as follows.
(defun external-tools--explore-here ()
"Opens Windows Explorer or Nautilus in the current directory."
(interactive)
(let ((dir (external-tools--get-default-directory)))
(when (fboundp 'w32-shell-execute)
(w32-shell-execute "explore" dir)
)
(when (and (not (fboundp 'w32-shell-execute)) (executable-find "nautilus"))
(external-tools--exec (executable-find "nautilus") "-w" dir)
)
)
)