Tim, mask it, then rough it up.
I cannot vouch for the mechanical bond that you will get with ultracote and rustoleum,,
The other way,, what I would likely do,,
mask your trim outline with 1/8 or 1/4" fine-line or other masking tape,, then mask outside that tape with another 1/4" ( depending on the turn radius you may need to use 1/8")
scuff the crap out of the ultracote inside the masked edge,,
pull up the original tape, and re-mask using the second line of mask for a guide( this will insure you get back where you started, and have scuffed ultrakote where you want it)
spray away,,
oh and make darn sure to clean the ultracote with alcohol, then wax and grease remover, then soap and water before you mask. mold releasing type agents can be very tenacious.
If you have cleaned the surface properly,, laid the tape properly,, not exceeded the bend radius of the tape, and apply your paint properly, you should not have any bleed issues,,
and of COURSE we know this effort isnt going into the worlds second most labor intensive Arf flight streak, ( Dirty Dan has the MOST labor intensive flight streak as I recall)