The only solution is to remove the degraded mortar to a depth of at least 15mm and re-point the brickwork with new mortar. Removal of the old mortar between joints must be done carefully in order to avoid damage to the arrises of the bricks. A hammer and plugging chisel are preferred to an angle grinder, particularly on very old buildings, where the bricks are comparatively soft, and under-fired in many cases. Very soft mortar can probably be removed with an old flat-head screwdriver. Where joints are particularly narrow, as is often the case on very old brickwork, it may be necessary to remove the mortar with the blade of a masonry saw. While a small-bladed angle grinder undoubtedly provides the fastest method of mortar removal, it is difficult to keep the grinder aligned with the joints and the chance of damaging the brickwork is very high, so raking out by hand should be favoured wherever possible.
When raking out, work downwards from the top of the wall and work on approximately three courses of bricks at a time. Always remove the mortar from cross-joints first, followed by the bed joints. If the bed joints are removed first, there is a risk of chipping into the bricks above or below when one comes to clear the vertical cross-joints.
The long-term success of raking out and repointing depends enormously on achieving a raked-out depth of at least 15mm and making sure that all the mortar has been removed from the edges of the bricks inside the joints. Failure to do this will result in poor adhesion and the new mortar being susceptible to the action of frost; it could simply ‘pop out’ after only one winter. Undertaking re-pointing but failing to rake out properly, or to a sufficient depth is, therefore, a waste of time and effort! Where mortar is found to be loose and degraded beyond a depth of 15mm, it should be cleaned back until reasonably firm mortar is located.