Best Primer for Mirror Mastic

Mounting an approximately 4' X 5' mirror on a sheet of baltic birch to mount in a steel frame as shown in the section below. The steel frame is painted flat black and I was planning to paint the reveal a flat black as well. My glass supplier, who will mount the mirror, said to 'paint' the baltic birch to provide a surface for the mastic. Reading the instructions from several mastic suppliers they warn against using paint, and recommend only primer. I guess the strength of the bond becomes that of the paint/primer to the wood, and paint is less tenacious than primer? Is there a primer (flat black) known to be especially tenacious or appropriate for mirror mastic?

Attachment 403880

Mirror mastic is a pretty tenacious adhesive. It will stick to most painted surfaces without issue. If you're concerned about it, scuff the surface with some sandpaper and brush off the resulting dust. I've hung hundreds of mirrors using mastic, and the only time one has let loose was due to the wall studs warping and bowing the wall significantly.

Nothing to contribute with respect to black primer but my experience is that mirror mastic eventually telegraphs though to the front side. I've seen it many times and in the case of my own bathroom I know it started to show itself in about 15 years. When I mount mirrors in rabbeted frames I use mirror tape around the edges. Even if it telegraphs through down the road it's hidden by the frame. That stuff has incredible, instant, holding power.

John