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I also read a similar result by other users regarding PETG's porous surfaces. You may coat the surface, but I doubt the resin film firmly stick there reliably. I tried epoxy resin to cover PLA printed vat before, it worked fine for 5-10 times printing, but eventually they are peeled off. You may try different approaches, but I'd recommend PLA (maybe 2-3 wall perimeters and 50% or higher infill) for the best and least time consuming solution. :-)