Over time, the PVC filler or trim on your windows can suffer from wear and tear. PVC filler and trim serve to weld together the joints of the frame to keep them together. When done properly, they prevent water from seeping through the trim and entering the home. If you notice mould growth or if it's evident that water leakage is present on your windows, it may be time to repair the frame with more PVC filler.
Use a utility knife to scrape away any rotted wood (if you have a wooden window frame). Scrape away any mould or other debris to expose the gap.
Apply wood hardener with a paintbrush to the frame and leave it to dry. This will strengthen the wood and prepare it for the filler application.
Prepare the filler. You can use expanding foam filler if the gaps are large or PVC trim, which will weld together loosened joints. Apply the filler in layers to get the best effects. Squeeze the foam expander out of the tube around the gap. Don't fill the hole entirely; leave a 1/16-inch gap to allow for the filler to expand and fill the area.
Sand the area lightly after it has dried with a fine-grit sandpaper or block to make it smooth and even with the rest of the window frame.
Fill any remaining holes in the same manner or apply more filler if the original hole or gap is still exposed. Wait for it to dry and sand it again.