I'm repainting oil based painted trim in my bath with latex.

I am priming the oil base paint with latex primer, and then applying latex paint.

Do I still need to sand/prep the surface before I apply latex primer?

I've already tested with denatured alcohol to determine that it's oil based paint.

I thought you had to use an oil based primer over oil based paint.
I've been looking at the new Smart Prime by Zinzer that's water based but acts like an oil based.That probably doesn't count.

Whether I have to sand before priming with the latex primer is the real question.

Yes...IDEALLY...do a thorough scuff-sand with 100-grit paper!
It's the BEST way-period.

Also YES...Latex primers are perfect over Oil paints (AFTER SANDING!). It's done all the time. You CAN use an Oil primer as well...AFTER SANDING!
>>> Obviously, Latex topcoats are ideal over either type of primer.

Sanding, in a sense, creates the "broom finish" that you need on sidewalks, so they're not slippery. Why don't you slip on broom-finish sidewalks??
Obviously it's because of the "surface-profile" that's created.

SAME IDEA for scuff-sanding a smooth finish Oil (or ANY glossy) paint. You create a profiled surface that your new Latex primer can grip VERY well. In turn, your paint topcoats bond very well to the new primer.

>>> Result??
* A paint/primer film that's bonded well, BOTH physically (from sanding), AND chemically (the paint-to-primer bond).
* Thus, you have a paintjob that'll stand up to the hard-knocks of real life!
* Make sure you're using at least a Satin-sheen paint!

Faron

And from dannylipford.com:

"While it used to be necessary to apply oil-based primers over oil-based paint and latex primers over latex paints, many primers today allow you to switch between them as long as you PREPARE THE SURFACE PROPERLY. When painting over interior oil-based woodwork with latex, be sure to sand or degloss the surface first, then paint with a BONDING PRIMER before topcoating with latex. Painting over multiple layers of oil-based paint on the outside of older homes with latex paint can cause adhesion problems, so in that situation, you're better to continue using oil-based house paint."

We used to tell people you couldn't paint plastic, either...technology changes.