Concrete floor coating is a serious business. Time and /or money are at risk for the installer/user and the vendor supplying the coating product. Both parties are tied indirectly to the ability for the floor coating product to adhere to the substrate. Failure is not an option.
Garage coating products (urethanes, epoxies, sealers, etc...) have to "stick" to the substrate (concrete floor) for a positive outcome. All the available products on the floor coating market share some facts. FACT 1: All floor coating products adhere to the substrate better when the substrate has an improved profile and porousness (excluding stains, densifiers and stainguards).
FACT 2: If the coating loses adhesion (peels, chips) it is probably the lack of preparation that is to blame, not the product itself.
This profile is measurable by the Concrete Surface Profile scale. This scale ranges from CSP 1 through CSP 9. See the scale below.
Manufacturers offer suggested CSP levels for individual products. In general thinner coatings like urethanes are to be applied to CSP 1 through CSP 2 level concrete. Thicker products can require a greater profile. It's important to ask the manufacturer what that recommended CSP is. In some cases an older porous floor may not require grinding as the floor is "etched" by time and traffic. This is something to discuss with the manufacturer. An acid etch can produce a CSP 1-2, a diamond grind CSP 2-3 and shotblasting CSP-3 and greater.
It is common for some vendors to recommend "no prep" or "no etching or grinding". These vendors are playing the odds that you may or may not have an issue. They are betting that enough people will purchase these products to outweigh the issues some will have. Why do they do this? Becaue most people are LAZY and want the EASY way out! When this happens you have become a unwilling participant in the "fools gamble". Don't be a fool.... prep your floor!!
Image from: http://shotblastinc.com/industry-guidelines/