Preparation between layers of glass/epoxy

At what point does epoxy resin (in a general sense) stop chemically bonding with the next applied layer of glass in a laminate?

I am curious if, after the laminate becomes tack free, there is some surface prep that can be done fairly easily and quickly to continue laminating without having to let it fully cure and start sanding it all over again and relying on a mechanical bond?

 

If you can press a fingernail into the tack free goo and leave a dent, you're probably safe to assume a chemical bond with additional no sanding coats. If you have no dent or need to press fairly hard to get one, you should probably wait until a full cure, sand and go for the mechanical bond.

There's no specific point, just common sense and experience. This is because of hardener types, environmental differences, thickness of laminate, etc. Once you get a feel for a brand or hardener under certain conditions, you can fairly predict when things will occur (working time, pot life, gel time, tack free, etc.), but it's still a crap shoot if you want real precision.

Think of it as the next application going over something soft enough, to be attacked by the subsequent coating (chemical bond). If it's too hard, you have to add some "tooth" to it. If it's soft enough, it'll tooth itself on a molecular level.

 

PAR is right. Epoxy cures slowly, up to a 100% cure. (already at some 60% or so the stuff is solid). Some epoxy cures easily to 100% (no chemical bond possible) but some stay at 80% or so, and need heat to reach their full 100% (some need considerable amounts of heat, actually).

It all depends on many variables, including properties of your epoxy, and temperature as main variables, at what time you can still overcoat without sanding.

The thumbnail trick is a good one. But also keep in mind that some epoxy can "blush", leaving a greasy substance on the surface, inhibiting any bond at all. So not only press your nails in the surface, but also run your fingers over the surface, and see if that leaves trails. If so, please let cure, rinse with vinager or water/ammonia mixture, sand and recoat.
Blushing occurs mostly in a damp environment, hot or cold does not seem to matter.

Another option is the use of peel-ply. Apply this as the last layer, and after peeling off, your surface is prepped for the next step (works in all circumstances, whether the epoxy is fully or not fully cured, or has blushed)

 

What kind of resin? They are thousands of formulations. Polyester laminating resin is air inhibited and will stay green for a very long time.

 

He says "At what point does epoxy resin (in a general sense)" so I guess he made his choice already.

For polyester resins, you are right, mostly. If you like to depend on a chemical bond, please steer away from LSE additions in the resin, or DCPD resins, which are more prone to secondary bonding failure then ortho, iso or iso-npg resins. Things have improved over the years, I must admit, but still be cautious.
LSE additives lay a thin layer of wax over the resin, which can decrease bonding.