most durable exit device

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The 99 and 33A have  grooved mechanism case and cover plates and the 98 an 35A have smooth mechanism case and cover plates.  There is no difference in durability between a 98 and a 99 exit devices.
Exit devices are not designed for exterior application. If used in an exterior applications then a powder coated finish on the 33/35 and 98/99 series are recommended for exterior applications. ...
The Von Duprin Pneumatic PN98/99 series exit devices feature provides remote latch bolt retraction in hazardous areas where electrically operated devices would not be permitted.
The Von Duprin Pneumatic or "PN" device was designed for Intrinsically Safe Class 1, Division 1 applications.
Recommendation is to use mild soap with a damp cloth to clean an exit device.