I have an on grade slab with black cutback mastic left over from a 1950s tile floor covering 95% of the house.
For height reasons, I want to glue down my engineered hardwood as opposed to floating a plywood subfloor.
I've read various product data sheets and called technical service lines for clarification.
One of the adhesives I'm considering is the Sika T55. In the data sheet, it says to remove the cutback "or use an industry approved leveling compound over the cutback residue." I called Sika for any manufacturer recommendations that will work well with Sika and he didn't have any / give any. Mapei said bluntly that they have no products intended to go over cutback.
I think I have 3 options and wanted to get your opinions on them:

1) Scrape off the cutback adhesive following the Resilient Floor Covering Institute's guidelines for removal.
2) Apply a leveling compound over the the cutback and then apply the adhesive to that.
3) Scrap the glue down; suck it up and live with the extra inch of the plywood, 6 mil poly, and 15 lb. roofing paper that floating a subfloor will bring. Buy a solid hardwood floor and nail it down to the plywood.

If you like option 2, can you recommend a leveling compound? Do you like the T55 or do you prefer others?
Thank you very much for your help.

You can use ardex feather finish and I have many times or the K-15. Make certain you follow the instructions explicitly.

http://www.ardex.com/cms%5CAssets%5CPro ... h-us-e.pdf

Or you can float your floor right over the cutback. I'm in the process of installing a floating locking floor over old cutback right now. Scraped and leveled a few spots around the house that were out of the spec, covered with 3-in-1 Shaw foam underlayment and now installing the floor.

ABSOLUTELY And thats one great reason to do a floater and why they made them in the first place.

I walk into a place with cutbac and old VAT or bad disintigrating concrete the first Thing I say to myself is FLOATER! Even though when pressed to gluedown over it I use the ardex feather finish. I am still rather worried that it will not hold in the long run.

I did a tile job over cutbac residue in my own house years ago, luckily I sold it but it all started popping loose and I used additive in my thinset and in my grout. And I tell ya that floor was scraped down to the concrete. This has always lead me to believe that any of these claims about some super duper proiduct that is designed to use over cutbac is way over rated.

Ditto, Here's my theory.... I believe the petroleum in cutback soaks and works itself way down through the pours of the concrete over time. Then you try to cover it, and the petroleum migrates back up through the concrete attacking anything it it's way . Like the creature from the black lagoon

Thanks for your responses.
Based on what you said and what I've been reading, I'll either go with floating or a plywood subfloor/nail down installation.