As I mentioned on another thread, I used a commercial product which contained ethylene glycol. Being hygroscopic it retained a little moisture in the FB print. It was very effective, I have several prints made over 40 years ago which are flat with no evidence of detrimental effects.. Since ethylene glycol is toxic, propylene glycol would be another choice. It is non-toxic, used for treating wood turning for salad bowls, etc. Amazon carries it, as does Rockler, a woodworking supply house. I'd imagine that Photo Engineer might have some thoughts on dilution, etc.