The idea of the spray is to fill any microscopic scratches etc (any of those made when scraping off the old gasket for example) ; it should not be needed if the block and head have been machined. If only the head has been machined I would suggest you at least use it on the block side of the gasket.
I see by googling that spraying with aluminium paint is considered a 50's , 60's and 70's thing and has been replaced with the purpose made sprays. from VHT, Permatex etc Makes me feel old. See
https://answers.yahoo.com/question/i...9203815AACkzIi ,
http://www.jalopyjournal.com/forum/t...askets.791868/ ,
http://www.460ford.com/forum/archive...p/t-97758.html http://sbftech.com/index.php?topic=14485.0;wap2 etc