When attempting to add a grant for all databases for a non-root user the following error appears:
mysql> GRANT ALL ON *.* to 'user'@'%'; ERROR 1045 (28000): Access denied for user 'root'@'%' (using password: YES)
This syntax implies every database/table, inclusing mysql.users and generally only root can see this.
Presuming that the mysql db is not needed and the user just needs full access to every databasem without the ability to amend grants the following should be fine:
mysql> GRANT ALL ON `%`.* to 'rmt'@'%';