Preserving the coins with broken patina

[vs1969]

Over time, I have got some coins with broken patina at the edges and limestone-like substance in the open, like the below Trajan and Nerva sestertii.  In addition to them being visually somehow disturbing, I have some concerns regarding possible further peeling off the patina, actually, while inspecting the Trajan sestertius recently, I have noticed something resembling the extra patina-free spot.

So my question is: could anyone suggest some measures to protect these problematic areas? Like applying some wax or some other substance?

[SC]

I have applied some Renn Wax to a few coins like that.  I brushed them first with a very soft nylon brush and then a silver brush. 

I don't think it actually help affix the patina but it is mainly to reduce friction if I handle it.

It seems to have worked a bit for me but I can't guarantee it.

Shawn

[vs1969]

Thank you very much for the advice!