hey doug
i have limited experience with liquid emulsion and plexi.
i used liquid light ... i didnt scuff the plexi but coated it with minwax urethane.
i had troubles getting it to stick, but it might have just been my inexperience
i also used gelatin as a sub layer with lackluster results.
my guess is that if you are using grit as you mention to give the plexi a tooth
you might be OK ... i have read of other people who did less than me ( prep wise )
and had great results so my poor showing was probably atypical ...
it might require more thwn one coat of emulsion, and use a hardener in your fixer bath
to make extra sure rhe emulsion wont frill / lift off.
just make sure your surface is extra clean and you will probably be on good shape.
oh coating .. ive flowed emulsion which is kind of fun ( think wet plate )
also used a foamy paintbrush with good results ( on glass, not plastic ),
and i have found if i free pour / flow the emulsion, the 2nd coat goes on much easier than the first ..
sounds like a fun project !
john