Polyester uses a free radical cure with the reactive diluent styrene.
Oxygen activates the inhibitor, so a wax is incorporated into the
resin-styrene solution. The wax migrates to the surface during cure
to prevent oxygen exposure and allow complete cure. Otherwise, the
stuff won't cure on the surface. You need to remove this residual wax
as it'll prevent adhesion of the epoxy.

Pete Keillor
Well-said Pete. I don't think sanding will every remove all of it.
Sanding could possibly just keep spreading it around. Use a wax
remover and don' look back.

"Quote from Awlgrip web site"

Awlprep Plus Wax & Grease Remover is used to remove wax residue and
grease from substrates. A medium-fast evaporating, strong, solvent
designed to remove waxes, oils, and greases commonly found on painted
surfaces, new or aged gelcoat, aluminum, or steel. Use before and
after sanding the substrate prior to priming. Always dewax before
sanding. Do not use on freshly applied primers or topcoats. It is
photochemically reactive.