Are the areas you need to remove right along the edge of the plastic? If so, a very sharp blade (Xacto, or a new razor blade) may allow you to trim off the bump.

Otherwise, very fine sandpaper and or a file should work. Scratches depend on the type of plastic you have, as well as the grit of the sandpaper you're using.

If you do get some scratches, they can be filled with something like Squadron Green putty and than gently  resanded

Hope this helps.

I have 600 grit wet paper, will that work? Do I use water with it or just the paper?

I'd hazard a guess that it should work with or without the water. I would test it though on a non visible piece before you dig in.

Sounds like you should paint it a concrete color, and not leave

in a bare styrene state. Having said that, begin sanding with #400 dry,

and finish off with #600 dry. Wash the surface with warm water and a few drops

of detergent. Let dry, and you should be ready to add a primer coat either

by spray paint or airbrush. If you spot any imperfections you can always resand

using #600 wet prior to painting. Get a sandable primer made by Krylon if you use a spray can.

Finish off with your color coat, and you're done.

Here are some progress pictures.

Most of my plasticard work to date has been with models that were handled constantly, so I'd be inclined to put some supports on the inside joints, even something as simple as a length of rod, but that's probalby not needed as this is to be a static piece.

it is looking very, very good!  One question though is on the bottom of the tunnel entrance. Is that section just there for support, will it be removed, or is it roadbed height already?

Thanks for you nice comments. The lowest 10 mm of the portal will be hidden under dirt. I'm waiting for squadron green putty from the US to fix my small mistakes before airbrushing it. Can someone please tell me where I can get very small letters for the tunnel? I think they are 7 mm and 4 mm in HO scale.

I will build the east tunnel portal soon, much more complicated with many buildings and a ventilation system + water tunnel.