> There’s also Rubio Monocoat and other two-component hardwax oils where the base component is usually a solvent-free blend of drying oils and waxes, and the accelerator component is Hexamethylene diisocyanate or HDI. The base component can cure on its own in about 3 weeks and the accelerator shortens the curing time to less than a day.

I find it bizarre that these finishes market the HDI component as an “accelerator”. It seems quite clearly to be a crosslinking agent — it’s a longish molecule with a rather reactive isocyanate group at either end. If you mix it with things it can react with, which likely includes both some waxes (those with hydroxyl groups) and some of the modified oils in “hardwax” oil, it will turn them into something akin to polyurethane.

Rubio Monocoat will cure into a different substance with the “accelerator” added than without it. In either case, it cures quite slowly and IMO has a nasty, penetrating chemical smell for weeks. I like how it looks, but the finish is not as stain resistant as many other options are, with or without the HDI.

P.S. the SDSes and some common sense suggest that this stuff is actually HDI oligomers, not plain HDI. The oligomers are rather less nasty.

P.P.S. Isocyanates are, AIUI, not persistently nasty, as they are too reactive. They react with water to form amines, and unreacted isocyanates will react with the amines to form polyurea, which is reasonably inert.

P.P.P.S. The “molecular bonding” stuff that Rubio talks about seems to be nonsense. The part A + part B mix will cure into a fairly hard and tough plasticky substance even if it’s a millimeter or two thick. Don’t do that — it’s not so easy to get the resulting mess off of whatever surface it cured on!