There's only a very narrow range of situations where it would make sense to use a leak sealer and be effective. I wouldn't use ANY sealer in a/c systems for the reasons mentioned above. You run the risk of potentially introducing more expensive damage to where the entire system would need replacement... much more expensive than the original evaporator issue.

Looking at the big picture, your least expensive (and least risk) option is to replace the evaporator and go through the vacuum and recharge procedure.

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95 E320 Cabriolet, 169K