Seam Filling "How-To" from George Gassaway

George Gassaway is a master builder and competitive flyer.
If you really want to see how it's done, check out his website at:
http://homepage.mac.com/georgegassaway/GRP/GRP-home.htm
On that website George also has contest tips and winning plans.

Recently on the Rocketry Forum he explained a seam filling technique that is too smart not to be shared. (Used with permission - Thanks George!)

"Here is what I do for models where I really want to get rid of the seams (usually only for scale models, I do not bother with the seams for most sport models)

I use masking tape to mask off the whole tube EXCEPT for the seam (if the seam has a 1/16" gap, then there should be a 1/16" gap between the masking tape on each side of the seam). Then use a good primer to spray a coat that will go only into that seam. Some builders like Plasti-Kote Sandable Primer--available at auto stores--as it is easy to sand. Others have success using Krylon's white sandable primer, which is more easily obtained but can have problems with clogging up the sandpaper too quickly.

I will note what I wrote the above (and some of the below) for a short article years ago, before they had to change formulations of solvent-based paints like Krylon’s sandable primer (and probably Plasti-kote’s), so beware how those specific primers may end up.

After letting the primer dry fully, apply more coats (letting dry fully between coats) until the buildup of the primer will be a bit taller than the seam recess depth was (in other words, slightly above the rest of the masked-over body tube surface). Then remove the masking tape and apply a couple of coats of white primer to the whole tube. The primer paint filling the seam should be visible as standing above the rest of the white primer on the tube. Use sandpaper to knock down the primer covering the seam and make it flush with the primer on the rest of the tube. Don't overdo it. When the seam is pretty much flush, give the whole tube another coat or two of white primer and sand the whole tube until it's uniform, no hint of the seam left.

Even if a person is using something else, and not primer paint, to fill the seam, the masking tape idea is useful. Whether using a filler compound, or putty, etc., use something like a putty knife, old credit-card, or piece of stiff plastic, to scrape off all excess so all that is left on the tube is flush with the masking tape (do this as you go, while the filler or putty is still fresh and soft enough to scrape flush). You can remove the tape before the putty dries hard, but do not sand until the putty is fully dried out and hardened.

BTW - Before using anything like wood filler or other compounds or putties NOT made for use with models, it would be a very good idea to apply some so a scrap piece of tubing and let it fully harden. Then sand to see if the stuff actually BONDS to the paper tube. I cannot recall what it was, but many years ago I used some type of filler that did NOT stick to the tube very well, so parts of the filler inside of the seam came out during sanding. I ended up using spray primer after all."

- George Gassaway

I'm going to try this on the Saturn V build!