I would absolutely scratch up the e-coat with 80 grit in the area your going to be using filler then finish off the filler with 180 and then 320-400 any raw plastic left around the edges then use a plastic adhesion promoter in that area before priming
DA the surrounding area with p180 to scuff it up, but you don’t need to remove the e-coat to bare metal. 80 grit is not necessary. While you can use 80 grit, DA 120 grit is just fine and can be highfill primed right over. Less sanding required on the edges where you don’t have filler. You can go to 180 over the repaired area if you like.
Also, I’m not sure why you’re bringing up plastic in this thread Jonas? No plastic (incl. PP, TPE or ABS) is ever e-coated.
E-Coat or electrocoating is only applied to metals. You can read about it here: [url]http://www.ppg.com/location/chinaindcoat/en/technology/electrocoating/howecoatworks/Pages/default.aspx[/url]. E-coated parts need to be primed before topcoat (either WOW or highfill). Some cheap/budget shops will save money and time by painting directly onto the E-Coat but this will lead to premature chipping and possible delamination.
Plastic (PP-E and ABS) bumpers are mostly supplied in a raw substrate format, and (unless they are damaged and require repair) simply require cleaning with a pre-prep solution such as Presta Scuff Stuff which is applied with a (old/well used) grey or gold 3M Scotchbrite pad and water. This is then followed with a plastic adhesion promoter and topcoat.
Some manufacturers such as BMW supply their bumpers pre-primed. In this instance, a light scuffing with a P600 or P800 dry pad is required followed with decontamination with prepsol prior to topcoat.