This question already has an answer here:
I created blank.master in my top level /_catalogs/masterpage directory and modified it to hide all of the SharePoint screen furniture down the left and at the top of the screen, by adding inline CSS with display:none in relevant Placeholders.
Then I modifield mypage.aspx in the Resourcing subsite to use MasterPageFile="~sitecollection/_catalogs/masterpage/blank.master
This works fine for me, all the SharePoint screen furniture is supressed, but other uses are still getting the stuff down the left and accross the top. Even after CTRL-F5.