I try to disable the right hand column for ALL pages except the product info pages. Or from another view : I try to add the right hand column ONLY for the product info pages.

My solution is to enable the two sidecolumns in admin, then to supress (erase) the display of the right hand column in tpl_main_page.php, and finally to create a custom layout for the product page with the two sidecolumns enabled.
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But, is this really the zencart way !!! ?