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All the improper repairs are removed from the brick and the entire chimney is cleaned prior to repointing. A treatment with limewater is applied to the entire chimney to rejuvenate the brick and original lime mortar. Lime water is produced by vigorously mixing some lime putty with water in a bucket, then allowing the mixture to decant. After about 24 hours the lime putty settles at the bottom of the bucket the clear water above can be carefully siphoned out. This water is now saturated with soluble lime.
When sprayed on the original brick and lime mortar, the limewater carries the "free lime" deep inside the brick and mortar and when it evaporates it leaves behind the free lime within the brick and mortar, This lime crystalizes within, reinvigorating and strengthening the brick and mortar. This is a technique utilized successfully in the restoration of prominent Frescoes and Historic masonry in Europe.
Note the extensive damage on the original historic brick. Even when Portland cement repairs are superficial the original mortar must be removed to a depth of at least 1.5 times the width of the joint in order to perform a proper and durable restoration.