Your question does not clarify if you are applying wet epoxy to a surface or applying something to a solid epoxy surface, but consider these points as you procede: There are two components to plasma treatment in ambient air. One is the UV generated by the electrode, and the other is ozone. I do not find definative work to suggest how important they each are, but have seen reports that the UV can have bulk effects and the ozone only surface effects. If the substrate surface exposed to the plasma is susceptible to UV degradation, the residual surface fragments could interact favorably with the adherand and increase adhesion. If the substrate surface is reactive towards ozone, it will become more oxygenated and fragments generated on the surface (alcohol, carboxcylic acids) can form covalent bonds with the epoxy adherand.

Be aware that there is a time factor involved. The effect of plasma treatment is transient - its effect changes over time after treatment. A low Tg material will change faster than a high Tg material, and direct surface contact will also change the surface (touching the treated surface by another, as in winding a web into a roll).

In air plasma resultant surface chemistry will depends on the active species produce in plasma for example ozone (in case of most of atmospheric pressure air dielectric barrier discharge) or/and atomic oxygen and other dissociation/ excitation products as well as how they interact with surface of material exposed. The production of active species as mentioned by Dr. Wertheimer, mainly depends upon various other factors like type of plasma, applied power, pressure etc. Optical emission spectroscopy can be used as non-invasive tool for identification of active species and understand possible plasma chemistry.