The balloons or glass bubbles would help in the sanding and getting it smooth but there is a trick to doing that.
Take some masking tape and put the tape down outside the seam to the distance/width you want the fillet then do the same on the other side. I run two strips of tape on each side of the seam. You will notice that I did a epoxy saturation on each side of the seam , I let it dry (24 hours) before putting the tape down , this way the tape sticks to the epoxy and when it is pulled there is no chance any wood will come up with the tape.
Now take the fillet material and put it over the seam , when a section is covered take an old charge card and use it as a scraper/spreader over the fillet material and continue down the seam in one motion. The card will smooth out the fillet while filling it and any excess fillet will move out on the tape. That excess can be scraped up and used where you ran out. After the fillet sets for a spell , before it gets hard , pull the tape and you will have nice crisp edges to the fillet and one smooth fillet.
DISCLAIMER.........
It is not recommended to do this but I do....... After the tape is pulled I put on a rubber glove and get some acetone on my finger and gently run my finger over the fillet ... that makes it really smooth. I heard that alcohol can be used but have never tried it.
Also if you do the charge card trick make sure the magnetic (black) strip is not the side that is spreading the fillet , it rubs off and will leave the black in the fillet. Plus if you think it will take two rolls of tape to do the job get three or four , you do use a lot of tape but it beats sanding.
Chuck.
Then you get fillets that look like this when the boat is done. If you will notice , at the bow ,on the inside , you can even do curves that way.