Uh, how about four layers of primer filler exactly where it needs to be, 3/32" wide or less, not 4 billion layers everywhere?
I do this:
Mask off the sides of the spiral seam. If the spiral seam is say 1/32" wide, the masking tape is going to cover along the sides, leaving a 1/16" to 3/32" wide gap, which has the 1/32" spiral gap in the middle. You are only going to put primer on THAT gap, not the whole tube. Well, by the nature of spray cans, you'll apply more primer than 3/32" wide, but the excess will be on the masking tape, not the tube.
Apply a coat of a GOOD spray primer. My favorite now is Rustoleum Ultra Cover 2X white primer.
I apply a nice coat, let dry thoroughly, then apply another. Ideally, one coat per day, for about 4 coats. But 2 coats several hours apart, for two days, is probably OK. By the time the last coat is applied, the thickness of the primer should have brought the "bottom" of the primer in the 1/32" spiral to be high enough to be above the surface of the paper tube. When you are sure it is thick enough, then about 30 minutes after the last coat remove the masking tape, and let it dry for another day so all the coats can try to completely dry (this is why NOT give 4 coats in one day). Sometimes I give a coat of primer to the whole tube and then sand along the seam area. Other times after it dries I do not give it another primer coat, I sand the seam with the tube itself still bare.
This method means you do not end up filling and sanding the h*** out of the whole tube. You fill the area that actually needs it, the 1/16" to 3/32" wide spiral of primer. And best of all ,you only need to sand DOWN that 1/16-3/32" wide area.
Now, after the seam is sanded smooth, then I do add another primer coat or two. For one, to make sure the seam area really did get smooth, if not it will show up and can be sanded some more in the problem areas. For another, the primer is a good base for the later "real" paint. As well, this ensures that the primer that fills the seam matches the primer that is on the rest of the tube. If a putty was used, like say a green putty used for plastic models, that dark green requires a LOT of primer to get to not appear as a dark spiral underneath. Been there, done that, never again.
I know it may sound like a lot of time...... but the only real "time" spent with the process is first masking the tube, then days later, sanding the primer along the seam. Taking far less working time than sanding and sanding and sanding a whole tube that has been covered with primer that is 95 to 99% unnecessary and sanded away needlessly. Now, it does take "days" between beginning and ending, but the in-between work is about 2-3 minutes to spray another coat of primer and let dry for hours or for another day.
So, if it is something you are planning for in advance, it is not an issue. If it is a rocket you start finishing on Thursday night, that you want to look PERFECT to fly Saturday, that problem involves a solution (intervention?) which is beyond the scope of this thread!
I have had hit or miss results with some putties. Sometimes, good. Other times, the putty did not bond well enough to the paper tube, so it chipped away. So when that happened it was worse than worthless. A few cases I did individual repairs, a few others I used thinner to remove all the putty and start over with something else. So, TEST any putty method on scrap tubing, to make sure that it bonds to the tube very well.
Also, some putties dry "hard" (Like a putty used for plastic models). If you have a hard putty on a paper tube, guess what happens when you sand it? The tube wants to sand more easily than the putty does. So either the paper tube glassiene layer ends up getting sanded rough (bad), so then you have to fill THAT, or the seam simply never gets sanded flush to the tube because you are afraid of ruining the tube surface (not what you want either). So, again, test first.
Final thing, back to the Rustoleum Ultra Cover 2X white primer. It is really worth it! Chan Stevens suggested it and boy was that a top tip! It fills very well, goes on thicker than most primers do. Sands very nicely. And....
A couple of years ago, I was using another brand of primer. The tube was a BT-60 type of white tube, that had a dark spiral in its seam. I used the method I've described, and got a nice smooth surface with no physical spiral. But I could SEE the spiral as a dark spiral color. The stupid primer was partly TRANSLUCENT! The Rustoleum 2X is pretty darned opaque, it fixed that problem right away.
I also found that the Rustoleum 2X primer also works well for filling balsa, not just for noses of course , but also fins. Now, that does involve a lot of spraying and sanding, but the results were better than other methods I was using.
Now having said all of that..... most of my models do not have the seams filled. I just do not bother unless I have a really good reason to want to do it for specific models. Like for say a scale model. Of course I have some sport models that were not even built until a Friday night, and I only cared about flying them on Saturday, not that they looked pristine, so........
- George Gassaway
