Now that you know that the culprit is most likely oil contamination, you need to clean and level the surface. I suggest using lacquer thinner to clean it, then using a scraper to level the surface. Sanding will work, but you risk sanding any oil residue into the surface. Scraping removes material without plowing anything into the surface. The best tool for this is a carbide scraper, which you can get at any home center. Look for one with a 2" blade for large areas and perhaps a smaller one for tight spaces.

When you apply your next epoxy coat, make it very thin. Trying to apply too much at once is the leading cause of runs, sags and drips. While slow hardener is great when you're applying glass, you may want to consider using a faster hardener for fill coats, as the longer it takes to cure, the more time it has to move.