Are both buildings needing to be cleaned? I would assume the contractor has already scraped the floor and removed debris leaving only the residue. If so a floor sander with a few 80 grit screens will clean it up. You will probably go through 10 screens and 2 or 3 pads. After it is sanded blow the dust out and wet mop it. Once it's dry use an airless sprayer to apply the seal. I did this axact same thing on my personal 30X50 building. My rational behind not covering the floors was because there was already so much trash and sheetrock dust and mud on the floor that it wouldn't matter. We always seem to cut corners on our own projects. After scraping the concrete it took 3 hours to have it ready for seal. Everyone who sees the floor loves it and wants to know what stain we used, which we didn't. I'm not saying it won't take work, but it isn't hard or expensive to remedy. If you want pics of the end product I can send them.