Using just primer on the balsa parts to fill the open grain, I have found, will show up under different circumstances.
If in the sun on a hot day, it will swell.
Cold days, it shrinks.
Same goes for the red laquer automotive spot putty.
I use the Elmers wood putty, then give it couple coats of Titebond II.
Woody I hate to argue these points, but I must as mis-information like that hurts everyones finishing;
First: Everything sitting in direct sunlight on hot or cold days expands and shrinks a little it's the nature of all Materials. Actually everything has a specific "Thermal rate of Expansion" that can be looked up for just about any material in the universe.
That said: I've been using Cheapy Grey Auto Primer (The Cheapest I can get my hands on) for over 20years now without a single adverse effect, or re-surfacing of Seams or Grain, when applied over Raw cardboard, Balsa or Basswood. That is, as long as i've used the 3 coat process discribed earlier. As a matter of fact: Using the 3-coat and sand primer method; i've had FAR fewer after many launch seam problems on models from T2+ Micro Maxx size to BT-101 and 5" OD .021" thin wall craft paper tubes. The "usual" Landing stress seam creases just don't show up when using the 3-coat primer method.
If you've seen swelling or re-surfacing of spiral body seams under your primered and painted rockets, then there is some other reason.. Perhaps only a single primer coat. or something causing the swelling. If you've sealed the surfaces well, there should be nowhere for the parts to absorb moisture which would cause the swelling, beyond the normal sun radiation expansion.
Using glue (any glue) to coat entire body tubes seems to me to be an extremely slow, labor intensive process, particularly if used on larger model. I'd think it would be comparable to coating an entire model in Pactra (Now Mid-West) Balsa filler. To be sure; I would not use such a process on Micro models let alone anything larger.
Can you explain the thinking behind coating bodytubes and/or wood with a wood glue? Glues; any glue shrinks badly, certainly doesn't fill body tube seams very well, I've on occasion used Carpenters Glue over a sealing coat of Wood hardener on Balsa, Basswood & plywood fins, Balsa Nose cones and transitions. Very occasionally but only on models with fairly tight grain as the glues take so long to cure between coats.
Auto grey primer on the other hand can be applied in 3 heavy wet coats in under an hour and depending on local humidity can be ready for sanding after curing overnight. These sandable primers work exceptionally well with 220 to 360 DRY sandpaper. I NEVER sand primer any finer than 360 to ensure I've left just a little "Tooth" to help the paint adhear to the primer.
ps: Never wet sand any primer on Craftpaper or cardboard body rockets.. Primers are by their chemistry porous, allowing water or moisture to pass all the way to the cardboard body tube and/or wood causing warps and delamination of the body tube. DRY sand primer only. Wet sanding should be saved for the Finial cured paint coat to smooth the surface and remove minute dust, hair or other imperfections.
