When I used 9 oz. seam tape, I always scraped the edges of the tape with a cabinet scraper. Otherwise you'll never fill them or get a smooth transition without a lot of sanding and more epoxy coats.9 oz. tape one inside and outside is actually overkill. You can sand down the outside tape a bit.

I like gravity and do mine on it's side. I use the fast epoxy for that and can do two coats in a day on each side which usually does it.