No bare patches. You need all exposed wood sealed in epoxy.

If you're sanding runs and getting to bare wood, you're doing it wrong. You need to sand only the high spots and leave the low ones  alone. You also need to stop sanding when you get down to the low spots, even if there are high spots left. At that point you need to apply another thin coat of epoxy and sand again. Concentrate the application in the low spots and the sanding in the high.

To sand only the high spots you need to hold the sanding pad level and don't let it follow the dips. As you sand the high spots should be matte and the low spots shiny.

When you apply a new coat of epoxy, make it as thin as you can. Lots of thin coats are better than a few thick ones.

Try doing it this way and it should go better for you. It's just a matter of practice, you'll get it eventually.

Laszlo

What Laszlo said.

To which I'll add: drips and runs are kinda inevitable until you get a handle on putting epoxy down right during application. Taking drips & runs down by sanding alone is why you're gnashing; it's almost impossible to avoid sanding lower areas adjacent to those drips & runs unless you're using what's referred to as a flat-board for the task.

InsteadI suggest you try a more focused technique whenever possible: use a scraper. This will ride the high spots until they're shaved down, 99.9% guaranteed to avoid doing any collateral damage to the lower areas you want to leave alone.

Scrapers come in all kinds of sizes & shapes. You can make your own from old files if you have a bench grinder. CLC sells 'em in several shapes & sizes.

I like using a single-edge razor blade held between the fingers and thumbs with both hands. Sometimes you can warp it into a shallow curve so its corners are less likely to touch the areas to either side of those drips. More often just dragged or pushed carefully while being held almost - but not quite - perpendicular to the surface you're working on is easier. Takes a bit of practice if you've never done it but the time spent is worthwhile.

Scraping will remove more epoxy faster than sanding ever will when you're dealing with high spots from drips and runs in the last coat you've applied.