Mud and caulk are not designed to stick to wood in the way you want to use them.

The best permanent repair is to remove the boards and repair the wall surfaces or add sheet rock.

By adding sheet rock and lathing you will be able to match the thickness of the boards and be able to reinstall all the trim.

The wood is always going to move. Any filler solid enough to produce an absolutely level filling of the grooves is destined to fail. Caulk all alone (first remove all of the joint compound) will fill the grooves sorta/kinda, but would be flexible enough to withstand the movement.
The middle groove of v-groove can be filled perfectly well with bondo, but the edge grooves (at the T&G joint) are the "mission impossible" culprits.
Casey

They are all T&G, Casey - individual boards. Do you think I could caulk the joints as level as possible and then use the wide Fibatape? Maybe some additional nails would help too.

No way am I taking down all the molding. This is a finshed house. If all else fails, and as much as I hate it, I will sheetrock over w/ 1/4".

Bill/Memphis

"Maybe some additional nails would help too. "

All the nails in the world will not stop the movement of the T&G wood as the humidity changes through the year.

It is not that hard to remove molding for re-use with little damage.

Wonder bars and some 6 inch drywall knives to help spread the force on the molding will remove it easily (putty knife in first, followed by wonder bar. Pry gently. It does ruin the drywall knives for finishing drywall.)

Any nails that come out with the wood should be pulled through the back if the trim to avoid damaging the front.

This appears to habe been something I never would have thought of. I dissected out a few joints yesterday, taking care to see what the soft mud was all about. Turned out it itself was not soft - it was like a bubble underneath. We floated over a layer of paint that was evidently latex over oil and it is pulling away from the underlying layer. I suspect that the moisture in the mud was drawn into the latex layer, loosening it. It is not a problem except in the grooves where the mud was deeper.

I know how to remove molding but that's only the start. Then you have to re-cope the corners (2-piece crown) and deal w/ the gaps created between the door jamb and casing. Just too much, and too much mess in a finished house.

I relented and am laying 1/8" Masonite over the panelling. Easier to cut around molding, easier joints, much less mess. We roughed it up so it would look more like old plaster. It will look just fine.

Thanks for your suggestions.

Bill/Memphis

Why would you not think this was original? Seems like a fairly credible wall for 1923. And original or not, it has its attractions....

KarinL

Well, let's just say I've never seen panelling like this in a craftsman-style bungalow around here, and the people I asked who I thought might know didn't think it was either. It looks '50s. But I'm curious - does anyone know for sure if panelling was around then, and used in this way?

Bill/Memphis

The house my wife grew up in has knotty pine/bead vertical planks. Her home was built in 1933 and they purchased it in'42. If the original owners installed it, it would have been between the two timeframes, but I know there is nothing behind the planking. It is also installed on the ceiling. The kitchen and bathroom is plastered.

Panelling of some sort has been around in many eras from prior to your house to 70s at least; I'm no house historian so I won't venture much more than that. But have you considered asking in the Old House Forum? Post some photos, get some feedback.

KarinL