We have pulled up carpeting that was glued down to the concrete slab in preparation to lay an engineered hardwood floor. I'm concerned about what the acceptable state of condition the slab needs to be for a glue down of flooring. There are trace remains of the carpet glue left; not even detectable as you glide your finger over the surface. As a test I was able to scratch it off with a screwdriver without too much effort.

Can the flooring be glued to this surface or do I need to totally remove any trace of the previous flooring adhesive? If so what would be the best approach. Scraping or abrading, dry or wet and if wet, what solvent do I use? What other steps beyond this do I need to take? I've tried to read through several hundred threads in the archive and didn't notice anyone with a similar situation so any help would be much appreciated. I gotta get moving on this because the "outlaws" are coming for Christmas!

Thanks for any help.