Is hardener required with Duralux marine enamel. I'm very unfamiliar with the enamel game.

No

Any ideas why after almost 5 months my Duralux paint is still so soft that I'm already going to have to repaint it.

Painted in high humidity without drying agents. When using enamel paints it's best to use a fast evaporating thinner such as MEK. in high humidity it's better to thin with MEK and using thin coats. In your paint gun use the smallest jet you can purchase and do a couple more thin coats. Problem solved. Hardener can be used but with hardener enamel paint tends to fish eye worse

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Paint tends to dry better <70 degrees. If you don't have your thinning chemical to your outdoor conditions, you're asking for failire. Especially woth enamel paints because the hardening agent is in the paint. Also with enamel it's okay to scotch bright it and clear it, with whatever finish you want. Just be light with the scotch bright. I've painted several wwII aircraft and pre 2000 de commissioned fighter jets. Everywhere from Southern California to flordia. If you use MEK in a low humidity environment such as socal. You will get a rough paint finish. Learned this the hard way after I painted an entire s211 Sao Marchetti jet

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This was in socal. Luckily the clear coat was manufactured thick enough to compensate for the fast drying thinner. Nothing can be replaced for a good paint prep. green scotch bright lightly over the final paint layer saved this show plane.