I realize that filling spirals is much akin to a religious debate, but would offer another solution. I do like to fill the spirals, especially on LPR/MPR builds - half the reason I build those rockets is to display them, so I want them to look good. HPR, I am not so worried about. For HPR, if I care enough, it is usually just easier to glass the tube. For me, I build LPR and MPR "to show" and HPR "to fly". I do fly my LPR/MPRs, but they are the ones that end up on the "rocket wall", so they spend more of their life as a decoration than flying.
I too have tried all the techniques in this thread. They all work. The question for me is how much time it takes to apply the solution, how much time it takes to finish (i.e. - sand) the solution, and how durable the solution is.
CWF is easy and pretty quick and it sands well, but I find that it doesn't stick in the grooves very well. You get a lot of spots where the CWF falls out or doesn't fill the whole gap and spend a lot of time sanding. Also, sanding it produces a huge amount of dust and clogs the sandpaper. I have tried the fix-it epoxy clay and it too leaves gaps and it is horribly brittle. I would stay away from that clay for any use on a rocket. I tried it for fillets once and they looked great, right up until the fins got bumped and cracked the fillet easily. I have tried George Gassaway's method of masking off the spirals (and, combined that method with CWF filling). That method works great, but it does usually require at least two iterations of prime-sand, so it is time consuming and sanding primer is still very dusty and clogs the sandpaper.
I like to use a medium cure epoxy and add a whole lot of microballoons. Most microballoons directions tell you to mix a 3:1 epoxy-to-microballoons ratio, but for spiral filling, I use a lot more microballoons. I am not looking for strength, I am more interested in making it easy to sand, but still being sticky. I apply the mixture to the spirals with a popsicle stick or a gloved finger swipe (always use gloves working with epoxy). Then I scrape the spiral with a razor blade, leaving the groove filled, but the tube with a very light coat of the epoxy mixture. Once it dries, a fairly quick sanding with 220 grit leaves the groove filled and the tube smooth. Pictures below are a quick demo I did just now using a cardboard tube that was the core of some wrapping paper. It was down and dirty and ugly, but shows what I am talking about. On a real build, I would be neater and more careful about the finish sanding. Hard to see in the pictures, but the "after" pic from the sanded end state is smooth. The epoxy makes the tube look wet, but it is dry and smooth. The nice thing is the epoxy mixture really sticks well in the groove and won't pop/chip out, even if you sand it hard or parallel to the groove, but the microballoons make it easy to sand (pretty much the same as sanding CWF), but very little dust and it doesn't clog the sandpaper at all. Anyway, this solution works for me, not sure if it will work for everyone, but it seems to balance all the problems of filling the grooves with a number of benefits.
I anticipate there may be a comment about epoxy being heavy. I agree that it is heavy, but I don't think the amount that ends up in the spirals is a significant increase in weight over, say, CWF. Also, the microballoons do lighten up the overall weight considerably. I would also point out that, unless the tube is very short, I would not use the quick cure (5 min) epoxy. The pot life is way too short for me to get the job done. I tried it and got about a quarter of the way down the tube before the mixture was hardened up. I just use the 15 min stuff and that tends to be just right.
