Oh some of the rubber seals are a complete bitch, especially when fitters/window co's give you ones that are too thick..
You can get a soap that it intended for mounting tyres that should help lubricate the seals, yet not have any ingredients that would attack aluminium or its powder coating (given that wheel rims are broadly the same thing) but if the seals are too hard work, consider asking the supplier for some slimmer ones. When pressing in I tend to use a wooden block edge that's been softened slightly on the sander, as it won't scratch anything; powder coated alu frames mark for fun