Tape & Jointing Help!

greensalad

Fill about a foot across the tape, so you've got a fair bit either side. Much wider than you'd think. Then sand ideally with an orbital. You need to sand more than you think. Because the fill will be so wide across the tape, you've got plenty of surface to sand off to make a very gradual "hill" to the thickest part over the tape, if that makes sense. And then it'll blend seamlessly. 

Too narrow a fill over the tape means not enough room to gently incline that sanding to the tape and that means it shows under the paint, as you've seen.

stuart45

If it's done properly, it will only need a light hand sand on the last coat.
Using taper edged boards means the joints should be be totally flush.
Only butt joints should be slightly.raised.