posted 06-21-2010 10:05 AM ET (US)
Warrior,In your conditions mildew must be completely eradicated in order to get it under control. From your description it appears to me that some mildew has survived your previous treatment and it simply begins reproducing when the conditions are favorable which is almost always.
Sunlight is a good mildew inhibitor but bad for gel coat. Try the water/vinegar solution recommended by Macfam to kill and remove the existing mildew then leave the boat uncovered in full sunlight for two or three days. Consider adding some cover vents to your cover to improve circulation. The vents are widely available at marine specialty outlets. You have not mentioned your cover material but some fabrics breath more readily than others. Covers that do not breath well are more suited to Northern climes. You may also want to consider placing a light or fan under the cover but if so be wary of the potential for damage from them.
Once you've totally eliminated mildew it is much easier to prevent it's recurrence.
Butch