It would seem that only one type of putty knife is necessary. However, the truth is that there are many different specialized putty knives, each intended for a specific job. Putty knives vary by shape, size and material, and all modifications to the basic design have a purpose. Though the number of types of putty knives may seem unlimited, the truth is that when you look at all putty knives, you will see that they can be easily divided into 3 main types. If you plan on working with a putty knife, it is important that you understand the differences between these types.
Flexible blade putty knives are commonly used for spreading soft material around. In particular, flexible blade putty knives are used to create a layer of putty around the edge of a window as it is installed. Flexible putty knives can also be used to spread other soft materials, such as plaster, joint compound for drywall, grout, or anything soft and wet that needs to be made even or flat.
A typical flexible blade putty knife will be about 0.02 inches thick. Flexible blade putty knives can vary in width, with ordinary ones ranging from slightly over 1 to 6 inches wide. Putty knife blades can come in a variety of lengths and shapes. Some are specialized to get to hard to reach areas, and some are simply wide and flat to spread materials around quickly. In addition to the metal blade, a flexible putty knife will also have a small handle made of wood or plastic.
Stiff blade putty knives have many things in common with flexible blades. Like their flexible counterparts, stiff blade putty knives will normally be between 1 and 6 inches wide. However, they trade flexibility for solidity, achieving this by being about twice as thick—0.04 inches, in general. This is to give them an advantage in their specialized intended purpose, which is scraping materials away. Obvious things to scrap include the same sorts of soft materials that flexible blades are intended to be used on, but stiff blade putty knives can even be used to remove mass quantities of dried paint.
Disposable putty knives make serious sacrifices in quality, and are not appropriate for most jobs. They generally have the same width, length, and overall shape as the other types of putty knives. However, the handle and blade of a disposable putty knife are both made from a single piece of plastic. This results in a cheaper putty knife that is not as effective overall.
Disposable putty knives do have one thing going for them, however. In situations where cleanup is difficult, or even dangerous, a disposable putty knife can save you a great deal of effort and time. For example, if you are working with a material that requires toxic chemicals to clean off of an ordinary putty knife, such as tar, you are probably better off using a disposable knife and throwing it away afterwords. This saves you the trouble and danger of cleaning a knife you would care about.