geophile pondered "Are there any considerations in putting glass on an already epoxied surface?"

To which I can only reply: no, other than doing everything you possibly can to give the new 'glass-in-epoxy the best possible chance of forming a proper bond to the underlying wood+epoxy substrate.

General practice is to build up reinforced epoxy in layers. Start with unthickened to coat the wood substrate, allowing time for the resin to soak as deeply into the wood as possible. Temperature plays a part (warmer is better as the resin is thinner, more able to penetrate) but if temp's too high it begins to 'kick' (harden from polymerization) before it's penetrated very far. So a balance of some kind between choice of resin / hardener and the ambient temp of the object to which the resin will be applied must be achieved.

Then it's suggested additional layers be added farirly quickly, so the later layers of resin + reinforcement can make a chemical bond to the initial coat while it's still 'green' or just partially cured.

If (for any number of reasons) this isn't possible, one must do a prope job of sanding the surfaces to receive further epoxy so not only are they clean and dust-free (once sanded enough and the dust vacuumed / wiped off) before more epoxy is added. The sanding ensures a mechanical bond, maybe even a partial chemical bond,  despite the passing of time. Sanding breaks molecular bonds at the surface which then ought to be open to further bonding with freshly apoplied epoxy (I think....)

Be careful not to add much 'glass cloth and epoxy to the point you make the board slot thinner. It's easy to add too much that later makes board deploying difficult, particulary where sand particles are present in the water you get out onto.

Laszlo's comments about microfractures from temperature changes are meaningful. Adding layers of epoxy to seal aganist water intrusion is a way to overcome this potential threat. Anything you can add to the fresh epoxy that serves to keep the new coatings from fracturing in tandem with what's underneath will be worth the effort.

If not actual cloth there are fiberglass microfibers you can use as filler to lend some thermal resistance to the new coats. Ought to be thinner than using cloth, even light weight stuff, and the fibers being randomly arrayed may be tougher than common biaxial-weave (90° fibers) cloth in this kind of application.