I run a Ferment on Premises and start a whole pile of kits each month in large (46 litre) primaries. I put sanitizer (maybe a litre) in the primary and swish it around trying to get good coverage on the 23 litre section. Then I spray the entire primary with sanitizer.
FYI, I use iodophor because the metabisulphite (ie campden) spray would bother me big time.
Steve