First of all, welcome to the forums

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It mostly depends what tools you have.
You can use super-fine sandpaper, the same that\'s used to fix cars when they get scratches. I\'m lazier, and just use an X-acto blade (the same that i used to get the details off). The trick is to work slowly to get a smooth surface.
If you\'ve chopped off a whole section and need to build it up again, then, yes, any type of putty will do the trick. I\'d use GS with a rubber clay shaper, but many will say that miliput has the advantage that it can be cut and sanded easily to get everything as you want.