prepping urethane

[edmorl]

I bought a brand new Xenon front lip, but it has some minor imperfections (rough patches here and there) how could I "prep" the lip to have it all looking smooth and even ? I am not planning on painting it afterward so it has to look good. The part is in urethane.

oh and I looked this up before, on prepping urethane but all the write-ups included painting it afterward so I'm uncertain if it's the same process.