Butyrate dope gases off for a long time- months and months. So, putting Concept over it unless it is very well- aged isn't prudent. Potential for cracking and/or crow's feet.
Over Nitrate, no problem.
PPG Concept Clear is wicked stuff for your lungs- a booth or outside air supply mask system highly recommended.
It adds quite a bit of weight , too.
It'll darken whatever color that it is shot over, but doesn't yellow.
Good Luck