I break,

The nitro in the fuel attracts moister like a magnet. If you have moister in your fuel there is nothing you can do about it but replace it. I do believe this is what your instructor means as any moister in the engine would burn out as good fuel burned through your motor when started. I have my fuel cap taped with fuel connections and fuel right out of my fuel bottle. I save the red inner cap to pug it after my flying session so that the nitro -methane doesn’t pull moister out of the air when sitting in my shop.