Primer changes everything! BENJAMIN MOORE Fresh Start is my go to…good on any surface, and excellent for priming over old oil paint. You need to prime oil paint prior to painting with new latex, or water-based, paint. For us, we’ve done so much trim work that over the last 8 years we have developed quite the technique. First wood fill holes. Second, sand with 120 grit to remove bumps from wood fill, and chisel out fill from corners. Third, sand with 180 to smooth entire surface. Then use paintable latex caulking to fill in any large gaps in the seams and along edges (i.e. where trim touches wall). For gaps larger than 1/8 inch (yes, it happens…sometimes!), you need to fill with drywall compound.
Next you need to prime. Once dried, the primed product needs to be sanded quickly with a 220 grit paper…this removes the roughness on the wood that the primer brings out. You won’t believe how smooth the trims & doors will be after this step! At this point, this is where we feel like throwing in the towel, and throwing out the sand paper, but we don’t. We work through the dust because your finish coat will look chunky and unprofessional if you skip this critical third step.