"So should I spray white epoxy as a sealer over the gray high build patches or just spray yellow base to cover? "
Yellow can be a bear for coverage, some will cover in 2 coats, others will take 15 coats.
Get a sheet of plate glass and spray the whole plate with your base. Take a sheet of cardboard and mask off 3/4" steps for each additional coat. Let it dry and hold it up to the light to see how many coats it takes to cover COMPLETELY.
I do hope you used hardener in your original basecoat, if not it may be easier just to wet sand it all off and start over.
Good luck with it.