If you fill in the body tube spiral, what do you use?
I use to use DAP DryDex Spackle (goes on pink, dries white). It was very easy to sand but didn't do a good job hiding the seam, so I switched to model putty. It is a lot harder to sand but does a better job hiding.
I'm a fan of Bondo spot glazing putty. You have to put it on quickly, and it makes everything a red mess, but it sands smooth and paints well.
I'll use either bondo or Elmer's Carpenter's wood filler (the yellow kind, not the Grey stuff - too sandy).
Bondo for my larger projects. The CWF I also trowel onto balsa prior to finishing with sanding sealer... zero grain after the sealer... and nice and smooth... unless I rush the job!
Bondo does a better job on the spirals overall, but you won't get high from sniffing CWF!
When sanding, the spirals, it's best (I think) to use a sanding block so that you don't sand down into the spirals...
(Although it may sound unusual since a lot of my rockets are flying dry lol)
I usually spray multiple light coats, then after drying, sand it nearly down to the cardboard (careful not to sand into the cardboard or further), then repeat. It usually takes between 3-5 painting and sanding sessions. Usually not more than 5.
It sands great, and takes very well to wet sanding.
I had good results with my Eradicator build using Rust-Oleum white primer and white gloss paint. I did not do much sanding. The body tube and plywood fins came out very well. Check out the attached photo.
I am building a 6" dia rocket using Blue Tube, and the spirals are pretty deep, so I'm going to try something new. Several rocketeers said good things about Super Fil on TRF, so I ordered some from Aircraft Spruce. I'll tell you guys how it goes.