This is a question I get every time we install an epoxy floor or polish concrete. My answer is you always have to fill them! Investing in your floors and not spending a few extra dollars on filling the joints is like installing tile and not grouting it. Joints hold dirt, water, oil, nuts, bolts and numerous other items that can be pulled out and damage your floors.
Now what do I fill them with? Ideally you need a product that will provide sufficient strength (hardness) to stabilize the sidewalls of the joints while at the same time allowing for maximum movement of the joints. Remember concrete expands and contracts with temperature changes. For best long term results, your joints need to be filled under the conditions that the building is going to be operating in.
With 20 years of experience in installing epoxy floors, I have installed different product types and manufacturers of joint compound. Without question the best product to install in your joints is a polyurea! Why? Epoxy joint filler has the rigidity to protect the sidewall of your joints but has minimal elongation and will split or separate from the sidewall of the joint as the concrete moves. Urethane will move as the concrete expands and contracts, however urethane is a very soft product and does not hold up well under traffic – especially fork lifts, pallet jacks and other wheeled traffic found in distribution and industrial environments. Polyurea, however, provides the best of both worlds. Polyurea has the rigidity of epoxy while having over double the elongation of urethane.
For your next concrete polishing or epoxy coating project, consider using polyurea and protect your valuable asset. As they say, if it is worth doing, it is worth doing right!