Chimney Cap sealant

My fireplace chimney cap is slowly disintegrating. I have used Vulkem116 (polyurethane caulk) to glue the bigger chunks back together and to fill in the major cracks but there is a whole spider web of smaller cracks forming. IS there anything I can brush on to seal the cap and possibly forstall its total disintegration by a few years?
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Two choices I can think of - either have it disassembled and regrouted/repointed with proper mortar, or cap the bricks with a copper or galvanized sheet metal cap, to at least the depth of the last deteriorated brick. If picking the latter, make sure there is a good solder or silicone seal between the stacks, if any, and the metal cap.
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