Engineered Wood Flooring - substance in grain

Hi,

After the installation of my engineered wood flooring, by a recommended fitter , we discovered a white substance in the wood grain (not ideal when the flooring is dark!) - we assume it is the glue that he has wiped into the grain.

I've tried several different methods on a spare length to try and remove it, and the only thing so far that has shifted it is WD-40. Is WD-40 ok to use? or are there any other recommendations?

Thanks in advance for your help.
Maz

 

Can you post some pictures

 

This is the worst area, but it is one most boards!