E30 Zone

I had to hacksaw off the old chrome bumper as the bolts had corroded and woundn't budge. i did a decent job but rubbed a bit of paint behind the bumper. I was going to just spray on red hammerite to stop it rusting as it won't be seen when the new bumper is on what do you think?

What do you call that type of paint treatment on the rear valence and under the sills? It's not smooth but sort of rough in texture and seems to be much more durable than the rest of the body.

Do you mean the brush on underseal?

If it's not visible bodywork and you're putting paint on it, do as much rust prevention as possible. At least use a rust preventing primer and rub down thoroughly before you paint. Then a top coat of smooth hammerite in a suitable colour will finish the job. Not much point putting on an underseal in a concealed area as its purpose is to stop foreign bodies piercing through to bare metal. Not going to happen behind a plastic guard. In fact underseal might be a bad move as corrosion could set in behind it.

the durable/wrinkly paint is called stonechip