The clear does not stick well to glossy surfaces.
So we rough it up, then drip in our clear to fill cupped inserts or other glossy low-spots.
Quoted from northerndude:
Toss some 400 on a smaller block and it’ll find some of he low points also I bet?
Use a hand sander (like a 10x4" drywall sanding block) rather than a powered sander, if you are new to clearcoating.
Sand the playfield, and you will quickly see the low spots.
Once you have the whole playfield sanded, take the paper, and by hand, sand the low spots (a block won't follow the curves).
Tilt the playfield in the light, there should be no glossy spots.
You can use a wire brush to get into openings and rough the hard to reach spots.
If the back of the playfield is still populated, tape off large openings, smaller ones will be fine: