The old silicone bead has spots of mould, I believe the proper way is to remove the old silicone first then apply new silicone to get the same thickness and outlook.
But I wonder if any thing bad can happen if I do not mind to increase the thickness (e.g. 3mm to 6mm) by applying new silicone on top without first removing the old one?
With the increased thickness, there should be enough surface for the new silicone to adhere onto (even though the old silicone may not adhere with the new silicone). But it would be great if something that I can do to prep the old silicone (e.g. degrease it, sand it, etc.) to make it good to adhere onto.
Sorry for amateur questions.