Thanks everybody for your suggestions. I like the look of the fingerboard so don't want to cover / change that. The lines are a plastic material, I believe, which is probably why the Sharpie wore off - between the strings, interestingly enough 
I'll try some of the stains etc. mentioned above. I don't want to go to the expense of having them cut out and replaced or a new fingerboard placed.

I'll try some of the stains etc. mentioned above. I don't want to go to the expense of having them cut out and replaced or a new fingerboard placed.
