Had the same problem. Inadvertently left the mower out in a rain storm and
it would no longer start afterwards.
Poured the gasoline out, and let the tank dry for a day or two.
Then filled the tank using a measured amount of "Dry Gas" or some such
Pulled the cord and the mower started first time and has been starting on
the first pull ever since.
The instructions on the can of additive say it will do no harm and in fact
may be beneficial to use a measured amount of the additive every time the
gas tank is filled.