So, I wanna squirt some Rustoleum trim onto some Ultracoat.

My understanding is that I just rough up the Ultracoat where I want the Rustoleum to stick -- I have some nice fine brass wool that should do an outstanding job for that.

But how do I rough up the Ultracoat where the Rustoleum will stick, without roughing it up outside of where the Rustoleum will stick and thereby leaving unsightly blemishes?  I can't imagine trying to mask and then rough things up.  Or is there some way of polishing out the scratches from the brass wool?

While I'm at it -- how do I mask things off so the Rustoleum won't bleed under the tape?  The usual prescription I hear is to paint clear on the tape edge -- but what to use for clear?

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)

Hi Tim,
  I don't know if it would do the job, but in the UK Solarfilm ( http://www.solarfilm.co.uk/ ) market a product called "Prymol" (Presumably similar products are available from other suppliers). It's basically an "Etch Primer" which can be used to increase the adhesion of paint to iron on films and also to provide a stronger fuel proof joint where film surfaces overlap. I have used it on solarfilm products and it certainly works although being electric fuel contamination was not a concern.

TTFN
John.

I just re-read Allen Brickhaus's Finishing the Olympus in Flying Models.  He makes no mention of prepping the film surface, although if I'm reading things right he's telling you to peel off the tape when the paint is wet, or at least not fully dried (!?!).

This calls for a test piece...