One of the most oft asked questions by homeowners when it comes to prepping their garage for an epoxy coating is the subject of filling contraction joints. These joints (sometimes called control joints) are the deep V shaped grooves that run from one end of your garage floor to the other. Some cement floors may have saw cut joints at a minimum of 3/16" wide and 1" deep. In a typical 2-car garage, contraction joints usually look like a big plus sign that divides your garage floor into what appears to be four separate slabs. When the time comes to paint or epoxy coat the garage floor, many people want to make these joints go away in order to have a seamless looking floor.
Filling the joint is not an issue if you are covering your garage floor with vinyl composite tile and isn't even necessary for garage mats or interlocking tiles. However, if you plan on painting your garage floor or doing an epoxy coated floor, you can potentially create some cosmetic problems with that seamless looking floor later down the road.