There are numerous ways to do this, but I'd do it this way.
Don't use 80 grit on plastic. Use 120 grit for reducing the the appearance of the texture. You get proper bumper filler, such as BMC (it's grey/black fibrous stuff) that will actually stick to the bumper. Normal body filler will too if your doing it rough but won't stand up to much abuse before cracking and falling out. BMC or similar has flex properties like the original substrate so imo better bet.
You might want to apply a surfacer to it to promote adhesion. High build primer comes in variety of colours, grey/ yellow being common. You can get it tinted though for ease of cover up. If your painting it black, grey h/b primer and black has excellent coverage properties. Rattle cans are ok if you don't have equipment but it'll never be as good as 2k.