Glazing (burnt on soot) is another matter....it is impossible to remove with a simple brushing. It is concentrated soot and highly flammable, it should not be left to build-up to dangerous proportions.
It can appear like someone painted the inside of your pipes with high gloss black paint.
The sweep may recommend a method of glaze removal.
This could be by using a chemical catalyst (like ACS spray) or by a special method called Ro-Kleening (RK).
Do not trust any product that claims to free your chimney of creosote with its use! Or states that the brushable residue will fall into the flames after its use. Woodstoves and Fireplaces never are straight up and down, they commonly have baffles, dampers and/or offsets blocking a smooth drop into the flames.
Creosote Removers are actually Creosote Conditioners, they only change the hard glaze to a brushable powder which must then be removed by use of a brush. All glaze or conditioned glaze, must be manually removed after ANY product or method is used to remove it.
The RK method of glaze removal can be accomplished in a time frame of 1-6 hours, depending on degree of glaze. It is performed with a specially designed mechanical head that literally whips the glaze from the tiles or stainless steel chimney without harming the chimney interior. There are two heads, one for masonry chimneys and one for prefab steel chimneys. Used in the proper application, no damage will be done to the chimney itself.
Only a trained and knowledgeable Chimney Professional can effectively utilize this equipment.