All Australian Heritage sash windows (Victorian,Federation.Art Deco etc) have been built will fine precise joinery ,cedar frames and quality Australian hardwoods for sills. These windows can be seen all over Sydney’s inner west, lower north shore If ,however your timber windows and sills were made cheaply and mass produced from the 60-70s on-wards, the majority of these window sills have been poorly constructed with inferior materials such as Maple and poor quality Cedar. This is not a good solution for a window sill and once they begin to rot, the rot can permeate through the whole frame and often the only choice is to replace the window. I have even seen sills made for some spiral balance windows made in 70s-80s that are flat with no bevel! Meaning the water runs back into the window, a poor design beyond belief .
Don’t be fooled by a young inexperience builder who tells you should remove your traditional windows and replace with new… In our years of thousands of heritage window repairs in and around Sydney, we have seen very few window sills that are in need of total replacement. This is due to the quality of traditional windows and the hardwoods used. Mostly the damage starts when they have been exposed and untreated with good preparation, primers and paint for years and years. The first and most common problem is dry rot on the surface; the second is wet rot from water damage. As stated it is seldom that these hardwood sills will need replacement, as replacement is costly and is the foundation of the window.
The solution is not to leave it up to the painter as he will just fill it up with builders bog which will fall out within a few years The solution is timber inserts and good flashing and your traditional window sills will be protected from the weather for the next 100 years. This flashing can be successfully painted leaving your windows with new looking sills.