Highliting Ink drawing

​Hi Marko,
I couldn’t find the binder on these pens (I’ve written the company and hopefully will get a reply soon) but, for now, I’m going to presume it’s some sort of synthetic polymer.  
“Adhesion” is an ongoing topic on the forum, for which I don’t think there’s always a consensus…  Here, perhaps overly simplified, is my view. 
Two layers adhere best when there is both (1) dispersive adhesion (in simplest terms, the upper layer flows and behaves atop the lower layer), and (2) mechanical adhesion (the two layers physically intermesh with one another).  These two types of adhesion become more or less important depending on the binders in question; and there are other types of adhesion…so it’s more complicated than I’m presenting, but enough for your question. 

I don’t believe gum arabic-based paints (watercolor, goauche) adhere well to synthetic polymers – they tend to resist, slip and lift; and, even if you can get them to behave, there’s not much mechanical adhesion.  Being indefinitely water-soluble​​ (gum arabic doesn’t cure or polymerize), gum arabic paints are more suceptible to lifting if they haven’t literally sunk/intermeshed with the underlying support or paint layers.  So while you could probably add​​ watercolor or goauche on top in the short term, I’d question the durability.  I would say the most secure option is switching to a synthetic polymer white ground or paint.  
There are tons of options, of course.  If you want the layer to not look to shiny or “plasticy”, use a high pigment load, matte ​white: Natural Pigments Tempera Ground, or the many excellent products from GOLDEN (Silverpoint Drawing Ground; Fluid Acrylics, Matte Acrylics or High Load Titanium White Paints; Absorbent or Sandabl​​e Gesso). 

Koo