Another thing that might work was years ago (when I did extensive print toning experiments) was sometimes the toners were rough on a print surface and be uneven, so (I don't remember the exact proportion) but I would mix Knox gelatin into solution, and dip the prints in to smooth out unevenness by re-coating... This would dry with a very fine, very slight texture that might hold lead... The entire film could be immersed, then dried, hopefully giving some "tooth" to the base...
Gum dammar would work, but can take a very long time to dry if thick (hours/days), and stink up the room, so a consideration... (I use this to recoat very dry/brittle vintage paper speaker cones, and it takes about a week to set and stop stinking when using the good art store product...)
Michael's suggestion to pencil retouch a little on the emulsion side bears merit if you have a base side that's ultra smooth, and you can pencil the first time around without removal...
Steve K
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