I think your main concern is not going to be the epoxy repair, but getting the mattress goo off. Duct tape would be a better field repair, since adhesive is easier to clean completely than the mattress goo. It probably had silicone in it, and you probably worked it into the wood really well, and epoxy won't stick to silicone, and won't soak into the wood if the pores are plugged with it. I would almost say cut out that section and replace it with a filler strip, because sanding is probably going to spread it around. At the least clean it until you think it's clean and then three more times with the silicone remover they sell at home centers.

Hopefully someone else has a suggestion on this.

m