Better to aply a light coat of etching primer first ( repeat:light!) but sounds like you bought a urethane primer......wich would be ok too...

Best is Plasti-kote zinchromate primer, really tough ! have had fenders in the snow for years and Zinc held tough !!! if done patch by patch a couple of spray can will do the job with you urethane primer after.....just remember: there is a 3 hours window of application after to spray the primer....after it won't hols as long....

Dan