Patching and Plastering - removing or skimming sand-textured paint

I have a couple of walls that have been painted with a sand-textured paint followed by high-gloss paint. I need to skim-coat them to match the rest of the room. I began skimming only to find that the sand texture was very uneven and was almost impossible to smooth, even with a long knife. One of the walls is long, and one is able to sight down the wall. I am concerned that if it is wavy it will be very noticeable. I do OK with sheetrock repair, but this is a real challange. Suggestions?

Scrape as much of the sand finish off as possible. You need to scuff up the high gloss paint for better bonding. Use all purpose mud to skim(not light weight) the first coat.

Would a deglosser be OK to promote adhesion? Also, other than weight, what's the difference between light weight and regular drywall mix?

I'm not sure about the deglosser. All purpose mud is heavier and contains more adhesive so will stick better and also dries harder. Light weight has less adhesive and dries a little softer for easier sanding.