Understand that the new floor covering will only be as secure as the surface it is attached to. For example, if you glue carpet to a painted concrete floor, the glue is going to adhere to the paint, not the concrete. If the paint is well-adhered to the floor, well, no problem. But what if it isn't, or if you are using an adhesive with mineral spirits (paint thinner) in the formula?
The same thing applies when going over old adhesive. No matter what you do to it, it is still a barrier between you and the floor. Some people say no problem; the new glue will stick to the old glue, giving you double the bond. Nice idea, but fuzzy math. You don't have a double layer of glue. What you have is an old, dried, worn-out, contaminated layer of adhesive with a questionable bond to the substrate between your new adhesive and the floor.
So, what do you do about it? The best thing to do is get rid of it, get it all off the floor and start fresh. I can hear the howls of protest already: It will take too long, cost too much, and who's going to pay for it?
What about skim-coating? Skim coating is acceptable, but you are still faced with bonding to the glue instead of the substrate. Most manufacturers of patch and self-levelers want you to remove the old adhesive to give their products a chance to bond to the substrate. They have the same view as adhesive manufacturers: "Our product's bond is only as good as the surface preparation of the substrate it's applied to." The stuff will stick to paint, old glue, even a dusty surface, but if the surface is not secured to the substrate, it's just a problem waiting to happen.
So take off the old glue. There are two ways to go about it: Liquid adhesive removers and Mechanical removal.
Liquid adhesive removers do a great job, but remember to read the instructions! For example, if the floor is not rinsed with clean water to get all of the remover off, it could remain in the pores of the concrete and attack the new adhesive, resulting in job failure. Your local supply distributor is the best source for support and information on how to properly use these products.
Mechanical removal sounds like work; that's because it is. Mechanical removal covers the tool spectrum from hand scrapers to shot blasters and all the stops in between, including the little trick to remember when walking out onto a sticky floor that allows you to deal with the old glue without losing your shoes.
Some friends were doing the common areas in a large nursing home, pulling up the old carpet and replacing it with Collins & Aikman PowerBond with the "Peel & Stick" back. The retailer planned to go over the old glue with a primer; that plan went out the window when the first section was pulled.
The old glue had to come off the floor before the C&A primer went down. The job stopped until approval was secured for the extra charges for the adhesive removal. Using a "Scrape-A-Way" disk on a buffer head, the old glue, the very-sticky-and-hard-to-walk-on-or-cut-due-to-its-elasticity glue, was removed, and it was night work to boot!
The solution to the sticky-shoe problem: spread play sand over the floor to cut the tack (photo 1). It works great, and play sand is cheap, costing around $3 for a 25-pound bag. Once the adhesive is cut off the floor, clumps of it will stick to the bottom of your shoes.
Great approach, but what then? Simply mask the bottom of your shoes with duct tape (photo 2). When you are done, peel off the tape and your shoes are clean.
Fun? Not especially, but it is an effective add-on service that adds to your bottom line.