One material used for many pre-war American OO models, especially locomotives, is sand-cast bronze.

As implied by the terms used, the mold used to make the part is actually packed sand, which leaves a characteristic rough finish on the finished parts. 

In addition, bronze is not at all a soft material. The only way to clean the surface up is by sanding it. To achieve a very fine finish you will have to sand the model with multiple grades of wet or dry sandpaper, working your way down from rough to fine grades. However, the tip of this post is for your initial sanding do as much of it as you can with your part safely in a bench vice and strips of belt sander sandpaper.

I picked up this tip from a natural horn maker. What is great about the belt sander strip is it is very strong and you can sand the part relatively quickly and vigorously using both hands. For curved surfaces such as the end of the Scale-Craft 4-6-2 tender seen in the photo the sander strip is ideal. Also, you can tear it into thin but strong strips for hard to reach places.

Some areas of course you cannot reach in this manner, which means you will have to use files, small sanding blocks, etc. A perfect finish can be obtained, but it will be slow going.

When I see sand cast parts that come to me all cleaned up (especially with tapped holes!), I know and respect the effort it took back on some cold winter’s night before the internet.