More importantly I wonder why it's there in the first place with plastic? Almost sounds to me you might need to polish the bore to get rid of any rough spots. You can do this with multitude of products including steel wool and a jag or polishing compounds. Remember your just polishing not cutting!
After each cleaning I always coat my barrel in a rust preventative to seal the metal pours. I use T/C 17 lube (each to their own)...but I'm not happy with it right now. Then next time I take it out I dry swab the fire a primer to burn any oils that might be present.
