Dry sand always. When you wet sand you lose a feel for how smooth you're getting the clear, making it much easier to burn through, which is very bad

Your acrylic and polyurethane clears just won't have the same look as diamondplate. These clears are much better suited for pre-diamondplate, as they look very similar to the factory finish. You can use these clears on a diamondplate playfield, but only for smaller spot touchups. For example, when I repaired the scoops on my Jurassic Park, I spot cleared with Varathane and you'd never know.

Don't get me wrong, you can get this stuff REALLY glossy, but it just won't have that glass look. For any large diamondplate touchups on the scale of something like this, you really have to go all out and do a full clearcoat in my opinion.

Krlyon Krystal Clear is great for very small spot clears. It's cheap, polishes nicely, but it's only draw back is it's not the most durable, which is why it's better for smaller jobs.

Varathane is better for medium spot clears.

For filling, it all depends on the size of the job.

For small jobs, like filling in a little chip here or spot there, I prefer either varathane from a pipette or super glue. If using super glue, have a paper towel handy. Tear the paper towel in half to expose the towel's fibers. Put down a little super glue, then dip the corner of the paper towel into it to soak up excess. The capillary effect should suck up some excess, which is good, because we want to build up in layers. Tear off the piece of towel and dip it back in. You have maybe 10 or 15 seconds before it dries so work quickly. It's seriously put down glue, towel for 2 seconds, take towel out, rip off part with glue on it, drip towel back in, 2 seconds, take towel out, let dry.

If you put too much super glue down at once, when it dries, it will have a frosted look to it.

When you have it built back up, lightly sand it with 600 grit, then 1500 grit, then novus 2. Super glue, if done right, can take a real nice polish.