You still have an old -style windows, t.e. Soviet standards? They were in the set of window sills collected from individual elements. Over time, cracks and gaps between its components appear in the prefabricated board. Such defects from time to time require cosmetic repairs.
First you need to clean all cracks from dust and dirt. Then, using a watercolor brush, process the cracks with olifa and leave for a while so that the Olifa is dry.
Prepared cracks must be filled with waterproof glue or oil with thick -plated paint. Previously, the cracks must be slightly extended, and, filling it with the adhesive composition, tightly press the parts to each other and fix it for a while.
If your window sill is not composite, but solid, then the crack needs to be cleared, and then cut to the entire depth. A wooden rail of a suitable thickness is inserted into the formed hole formed. When the glue dries, the windowsill is cleaned with an sandpaper, oliphaty, and then painted.
In small cracks on a whole window sill, the inserts are not needed. It is enough to clean them of dirt, pouring, putting them with oil paint.
If the gap arose between the windowsill and the bars of the window box, then it is also sealed with putty on the basis of thick oil paint or a special putty is used, and then painted with paint.