There are times when you have the perfect bit in mind except for the fact that it's got some iconography on it that you don't want. The question then becomes, "How do I remove the iconography so I can use the bit?" I run into this problem all the time when building models and I figured I'd share the trick I use.
This technique will work on shoulderpads, greaves, or any other spot on a model that you can get to with your blade and file. I'm using a shoulderpad for this example.
You're going to need two tools. A fine tipped X-Acto blade (with a new blade of course) and any files you may have. The new blade on the knife is essential since you are working with fine detail and usually very close to your fingers. You want to be able to cut through the material as easily as possible and you don't want to be trying to do this with a dull blade.
I'll be honest, I don't use the files often for this kind of thing. If I do use them, it's on metal models. The reason I skip them on plastics is that there's the potential to scar up the surface too easily and leave it rough instead of smooth.
If you do go with a file, keep your pressure light and consistent. That and vary the direction of your strokes so that you don't end up cutting unwanted grooves into your bit.
When you start out, work from the top down towards the surface you want to keep. In this case, the skull sticks out the furthest. I'll cut that away by taking little chunks out until it's gone. Then I'll start on the wheel behind it. When you go to cut... CUT AWAY FROM YOUR FINGERS. This might make it harder to do, but it's essential. One slip of the blade and you can end up getting stitches. Take your time and cut away from your fingertips.
The other thing you can do is slowly slide the blade forward as you cut. This will help it move through the material and make the end of the cut less explosive. If you keep a nice, steady pressure applied, the blade will move right through the material and stop when done. It won't blow out the other side as it finally cuts through.
Here's what my shoulderpad looks like after I've chipped away at one side of the icon. I started with the skull and then moved to the wheel.
Once you have the majority of your icon cleaned away, you can finish it off and get the surface really smooth. This part is similar to cleaning mold lines off. Take your blade, angle it slightly and scrape away at the surface until the hills and valleys smooth out completely.
This will take a few minutes as you work from various angles. Take your time and don't try to scrape the surface smooth in one pass. A number of lighter pressure passes are far better in the end.
Often you will have some form of iconography at the edge of the bit or overlapping another portion you'd like to keep. In this case, I want to remove the extra tab in the corner of the shoulderpad.
The trick is to make small relief cuts where you want to mark off the material to be cut away from the part that is staying. Once you have your relief cuts in place, gently scrape or cut away the excess material. You may need to do this repeated times for things that are thick. Make a cut, scrape away... repeat as needed until you've removed all the excess junk you don't want.
As you get more comfortable with doing this, you'll find that you can remove more and more material that allows you to use bits in any number of ways. The modified shotgun above is a perfect example. I cut away the arm that wrapped around the stock. I left the stock there as I did it because I needed that particular bit without the arm.
Make sure to check out these posts as they might help:
How to remove mold lines from models
6 tips for sculpting shoulderpad icons