There always seems to be a lot of questions surrounding body fillers and putties so I thought I would put together a thread to cover some of the basics. I will give some general info about fillers and putties make up, differences, etc. and then a few tips. I apologize if some of the pictures aren't the best as I was making some panels for work and just did the stuff for this quickly while I was working on those. Keep in mind that all these products are designed for the Collision Industry and are formulated to increase production and reduce problems not for ease of use for hobbyists. Pretty much all body fillers and putties in North America are what are known as lightweight products. They consist primarily of 3 components: Polyester resin, Talc and Microspheres. Different companies then add different components to increase performance aspects but all contain these three basic components. Resins: The vast majority of fillers and putties use polyester resins. There are a few exceptions where hybrid resins are used but I won't get into that here. Generally more expensive fillers will use more expensive resins that will flow better and sand better. Putties use a unique resin that cures to a tighter and denser film which provides a more stable and more solvent resistant surface which is why they were developed to provide a better surface for primer application than what most fillers can provide. Talc: Talc is the main body of a filler. Think baby powder as a common talc product. The are different sizes and shapes of talc used in various products. There are two basic shapes of talc used in body filler and putty: Spherical (top) and Platelet (Bottom). Round talc is what is used in body fillers. It is easier to sand due to the fact that you are generally sanding a smaller area of the mineral. Platelet talc is usually used in putties because it is easier to spread and gives a smoother finish and a denser surface structure. The easiest way to think of the differences is think of trying to cover the bottom of a box with balls or sheets of paper. You will get more spaces between the balls than the sheets of paper and thus not as dense a surface. Combine this with the special resin used in putties and you get a surface that does not absorb nearly as many solvents as body filler during primer application. Microspheres: Mircospheres are tiny hollow glass or plastic bubbles that are added to fillers and putties. These are what makes a filler a lightweight product. Micropsheres main purpose is to increase sandability. Cutting through the hollow sphere is much easier than the solid talc particles. But there is obviously a trade off in they are not a solid particle that will add stability and density to a surface like talc will. Filler companies use this to their advantage to make cheaper fillers that sand really well without having to use more expensive resins and finer talc grinds by adding more microshperes which sand better but don't provide as dense or hard a surface.