I have a urethane bumper that was recently painted. I accidently got some scratches on the front of the bumper. The body shop guy said it was safe to wax within a week of painting it because of the paint/primer he used. I waited about a month and used Mother's power polish with a high speed buffer on the scratches, then waxed it. The scratches weren't as noticeable after doing this.

Then today (2 weeks later) I noticed that the scratches were more noticeable than ever. They look like thin white lines. What can I do to blend these scratches away for good?

depends on how deep the scratches are. When you run your nail across the scratch, does it catch? Depending on the size and visibility of the scratch you may want to see if your painter can simply blend that spot with some more paint and not having to paint the entire panel. Also, if the paint you shot is a factory paint, ie: duplicolor, they sell repair kits at auto zone which is a repair kit, comes with glazing putty, some sand paper, the base coat and a clear coat. If used corectly, you can almost vanish the scratch for about 20 bucks. But if you arent really experienced and dont take your time, it might become more visible than what you started off with. If you choose to use the duplicolor, take your time, and be sure to apply a few extra coats of clear to the spot you are touching up so you can try to buff it out as best as possible. Good Luck!