I am installing a 16″ x 16″ Armstrong MODe vinyl tile. I can’t install new underlayment over existing because of height issues.  Most of the underlayment under the old tile is in great shape except for a few spots.  The original plywood used was 1/4″ fir plywood.  There are spots where the old tile pulled up the top veneer and like fir plywood it wants to splinter more in some places.  I believe the underlayment will work as long as the patching will bond sufficiently to the existing.  I am using Armstrong’s S-194 leveler compound mixed with a latex additive.  Would it help the bonding to prime with some of the latex directly onto the flooring and old adhesive first or would there be issues between the old mastic and the latex.  I’ve used latex to bond joint compound and plasters to walls but this application is different and the floor installation expensive and I want to get it right the first time.  I would appreciate any help or feedback on this.  I believe the compund will bond but I had a problem once about 10 yrs. ago where the patch didn’t bond in one place and I couldn’t just reglue to repair.  It turned into an expensive repair timewise.  I really want to avoid this problem again if I can.  Any ideas or feedback would bevery much appreciated.  Thanks!