Easily order an extra hardener for a putty here. A polyester hardener has a distinctive colour so that you can very easily see whether the hardener is nicely mixed with the putty. A two-pack polyester putty is always mixed with 2 to 3% hardener. To measure this exactly, you can use a scale to weigh the ratio. During mixing, it is very important to avoid air bubbles in the putty, which you can easily do using a putty spatula and mixing board.