Never used Rocket-poxy..
But I have used various filler in various epoxies.
My go-to now is West system & their faring filler 407. The West system has a bout a 3 hr working time, so there is no rush! It seems to lay nicely, smooths out, and sands easily. Other will say to use colloidal silica, but I find that tends to be like trying to sand a rock!
I also now tend to use a plastic card; a hotel room key-card. Epoxy don't stick to it, they re free, and you can cut different radii in them. I also find they scrape epoxy away on a fin, instead of getting another 'build up'
Mix up teh epoxy, then add a few scoops of teh filler until it's fairly thick: peanut butter or cake icing... Then dab on. do take care to ensure you put it where you want it to go! (I see some people use a table spoon to slather it on, then spend hours sanding half of it away!) I then draw my key-card over the fillet, first at a shallow angle to work it in, then gradually bring teh key-card to near-vertical to get consistent fillets. And wipe the card after each pass! I find any bit will go here you don't want! and teh little 'seam' that appears can be nudged into or off with the clean edge of teh key-card.
Pull the key-card along the fillet, and draw it off, a the same angle off the tip of the fin. This tends to round it off nicely, but it does take a bit of practice, and requires a fare bit of clean up (again, always have a piece of paper town with you!)
Once cured, I then will sand with a folded over piece of used sand paper. it's also rolled to match the fillet. A few light strokes to blend it all in.
And, a bit of Bondo at the end, to fill in the few holes & divets. This is the most tedious part; filling in / faring in to get a nice tube to fin tip transition. At this point, it's a roiled up piece of sand paper, to about a 1/4" diameter, and a bit of a rolling action when sanding it..
go to post #22, to see see what I'm talking about (and to repeat what I just wrote!)
https://www.rocketryforum.com/threads/madcow-super-dx3-patriot-a-build-thread.140215/