Subframe bar and PP-plastic bumper treatment

My subframe is sprayed in matt black. I want to mount it as soon as I'm able to disassemble the complete subframe in front of the car. That'll last some time, for my bumpers are to be painted first. Next week I'll fetch the plastic primer and the color paint. And lots of sanding paper in tiny core.

Matt black is the colour for this element

Underneath the car, no one will see a red subframe

The bumpers are roufh on the surface because the plastic is untreated. I reckon they were painted simply on the plastic, without a prime layer of filler. 

Front QV bumper

Rough and untreated surface

smooth on one spot...

... rough on the other

The flaps for the foglights are different in surface, 

left hard and sandable, right soft and difficult to treat with sanding paper.

The primer will make it easier to sand and smoothen the surface.

Bumper sanded

Detail of the sanding of the bumper corner.

Primer and filler will make this part smoother too.

I'll keep you posted