Undoubtedly, these fettling knives are the most effective tool for any DIY work at home. So, while choosing a knife, you need to decide on its purpose of using. However, the flexible blade is ideal for applying the materials and the stiff one for scraping the materials. Thus, these knives are used to scoop up a small ball of putty with the flat surface of the knife for filling the gap. Besides, you can also use it for moving down the wall. You need to hold the knife at around a 30-degree angle above the hole. So, you need to touch the edge of the knife tightly against the wall. And make sure that the putty covered is on the bottom and bring the handle down towards you for easy moving.