I used paint stirrers to form a corner. I then double checked to make sure there was no left over loose debris. Wax paper was then placed on the back side and up the sides where it was stapled in place. I figured the staples would leave little holes that could easily be filled. I used wax paper because it won't stick to the epoxy and can be peeled off after the epoxy hardens.
Before opening the can...open a window. The fumes are intense. I mixed the 2 part epoxy according to the instructions. The instructions are kind of vague. Use 1/4 can of filler to 1/4 tube of hardener. That's great if you need that much filler. So you really need to guess the amount of filler and hardener based on the ratio of 1/4 can to 1/4 tube. The trick is not using too much hardener but at the same time using enough. The more hardener the quicker the filler sets up. I prefer two part epoxy that uses a 1 to 1 ratio. Paper plates and plastic putty knives are my choice of tools when it comes to mixing epoxy. When all is done, you just toss the plate and if you clean the putty knife off with a paper towel before it hardens you can reuse it. It is very important to completely mix the two parts. Without the hardener the filler will stay soft, so mix well. Once the filler and hardener touch.....don't dally. You will notice that the filler will start to thickens. Don't answer the phone, don't change the radio station, and don't take a break. If you do, your putty knife will be permanently attached to a grey blob on a paper plate.
Using the putty knife, I pushed the filler into the void and leveled making sure to push it into the opening. I slightly rapped the end of a screwdriver on the side of the sash to coax the filler into the hole. Know that you cannot get the filler completely smooth. It is better to have it slightly higher than the desired finished surface. After the filler hardens you can use sandpaper to smooth and level.
Tonight when I get home (3am) I will remove wax paper and inspect the back of the sash. I am sure that I will need to fill some voids on the back side.
Time invested...20 minutes last night and 1 hour today. Total so far 3 hours and 20 minutes.