Plasti Dip is, essentially, plastic in a can. It’s very versatile in wet plate collodion. It works very well for backing clear glass ambrotypes. It might need 2 coats but you’ll get a smooth very mat black finish even if it’s applied blotchily.
You could even black your plates before shooting them because it resists the chemicals and doesn’t appear to pollute the silver nitrate.
It’s great for protecting the inside of holders. On wooden holders with slides that bend it solidifies the bendy parts on the more fragile holders.
You could fix pinholes in your bellows with this too although I admit never having tried this.
If you make a mess, no problem, it just peels off.