Update - add'l info from Ardex (SLC) web site.
I've been researching SLC's to go on this concrete floor. Seems that Ardex is popular. I searched their site and found a concrete floor prep document that's worth noting below.
Basically they're saying that mechanical methods are best for removing contaminants and that sanding equipment is ineffective. I have to disagree with them based on the results from the particular equipment that I used. I think the problem for most DIY'ers is that we don't have the expensive equipment required to do this in the best possible way. Grinders, shot blasters, etc are probably pretty expensive.
I also wanted to point out that it may be important to test the glue residue or cutback to see if it has asbestos content before sanding. The sanding did kick up clouds of dust and I was wearing a mask. I did not test the glue though but I did test the glue under adjoining room's linoleum floor awhile back and it was negative.
I'm still struggling with whether to SLC or not. I think it needs to be, but it seems to require finess/experience that I don't have. I might just hire someone to do it and be done with it.
Below from Ardex web site -
RECOMMENDED METHODS FOR PREPARING CONCRETE SUBSTRATES
The best way to remove any contamination in a concrete substrate is by an approved mechanical method. Mechanical cleaning removes the contaminant and the concrete to which it is adhered leaving only a clean, sound and solid surface behind. Ardex recommends that all concrete substrate preparation proceed using one or more of the following mechanical methods: shotblasting, scarifying, grinding, sandblasting, scabbling (bush hammering), chiseling, and in some cases, high-pressure water blasting.
Mechanical abrasion methods such as scarifying, scabbling and chiseling are aggressive methods recommended to remove unsound areas. Grinding can also be effective, but is slow for large areas. Sandblasting is an excellent method for cleaning weak surface areas, if environmental restrictions permit its use. High-pressure water blasting may also be effective for some weak surfaces; however, the concrete must be allowed to dry before proceeding with any patching or smoothing work.
One of the most cost-effective methods for removing a wide variety of contaminants from a large area of concrete is to use shotblasting. Using different sizes of steel “shot”, a shotblast machine can remove a variety of sealers, coatings, curing compounds and other contaminants quickly and effectively, leaving behind a surface ready to receive the specified ARDEX underlayment or topping.
The best way to remove most contaminants is by scarifying, shotblasting or similar mechanical method. Removal must be deep enough to eliminate all penetrated contaminants.
Methods to Avoid
Acid washing (or acid etching) is not recommended because it is difficult to control, to fully remove the residue, and to properly neutralize. Further, the acid can penetrate into the porous concrete and chemically react with the cement, thus affecting the long-term integrity of the concrete. Acid washing will not satisfactorily remove grease and oil.
The use of sanding equipment is not an effective method to remove curing and sealing compounds.
All types of solvents should be avoided. Their use will drive oil, grease and other contaminants further into the concrete, only to permit their release back to the surface at a later time. Physically removing oil-contaminated concrete is the only sure way to ensure a clean substrate.
Sweeping compounds can leave an oily or waxy film on the surface of the concrete. Their use can create a bond-breaking layer that will result in a flooring system failure. Using a clean dry broom, sweep and vacuum the surface prior to placing any underlayment or topping.
Also, never use adhesive removers or solvents to remove contaminants from porous concrete. These materials can carry contaminants into the pores of the concrete, which will later migrate back to the surface resulting in a floor covering bond failure.