I wouldn’t put anything on the top surface as it is bound to show through. I would use the method matt described but only use a very modest amount of epoxy to avoid too much squeezing through to the surface. When you rub down & re-varnish the joint should be near invisible.
If the ply panel is flexing then strengthen it from underneath, a ply patch epoxied over the whole panel would be easiest to fit rather than using glassfibre upside down. If the panel is rigid it would confirm that it is only the outer veneers that have delaminated.
Regards Peter W3035