That's a new one on me - I don't recall ever coming across anyone recommending a felt gasket of some kind to go underneath the cone on the wood ledge. Did it sound okay before? Any perceptible buzzes or something that might be a little bit "off?"

OMI Dobros have been known at times  for shall we say, "inconsistent craftsmanship" and if it's really that rough of a surface I would think some sanding just to get it a little smoother would be the ticket. 

The pressure from the tension screw attached to the spider and downward pressure of the strings normally makes for a firm connection to the cone ledge.

FYI - depending on the height of the palm rest, be aware that the raised "lip" on the edge of Legend cone makes it sit up slightly higher than other makes of cones. It can make for a tight fit underneath the palm rest and might necessitate a bit of sanding on the bottom of the bridge inserts. 

As an aside and normally used for pre WWII Dobros with low palm rests, Beard sells gaskets that raise the coverplate slightly if it's too tight a fit for the bridge inserts: 

https://www.resophonicoutfitters.com/product/BCR-V.html

Vintage Nationals with biscuit bridges are a different breed of cat than spider bridge resos and I don't have any real knowledge about those. I wonder of modern metal body Nationals include a gasket under the cone? 

Any soft material between the cone and the cone-shelf will damp the vibration, reducing the treble response and shortening sustain. Most builders insist that the cone-shelf be level, flat, and free from any finishes or stains. Likewise, the outer edge of the cone should be flat and level, making full contact all the way around.

Some have gone so far as to harden the surface of the cone-shelf with wood hardener or super glue, allowing it to dry before installing the cone. This further increases the edge and sustain.

If the fit of the cone is precise, any rattles or buzzes should disappear when the strings are brought up to tension.

Don't use any "gasket" on wood-body guitars. Vintage nationals (particularly tricones) had kerfed carborundum paper under each cone, I suspect to minimize the possibility of buzzing. I have assembled tricones with the original gaskets, with felt gaskets, with very thin T-shirt fabric gaskets and with no gaskets. All of them work fine. The very best sounding one had felt gaskets however, no two guitars are the same so I wouldn't necessarily take that as proof that felt is the material of choice. Conclusion for your application = no gasket. Some of the 80's Dobros had pretty rough cone ledges so make sure yours is solid, flat and true.

It makes sense to me what Sam wrote about  material under the cone edge  will damp the vibration. 

I guess that's why  I and I'm sure most of us have never heard of doing such a thing.  

Who are these people on the internet who suggest it as an idea for a spider bridge resonator? 

Thanks to everybody for this, very helpful. I don't have the guitar in front of me but I think there is finish on the ledge the cone sits on, but the whole thing looks like it was built in a middle-school shop class. The '29 tricone had the original cones and thin cardboard with printing on it, maybe it was material between sheets of sheet metal to protect if from scratching during shipping. And the biscuit cone had felt, but there is a big difference between wood and metal bodies. The new cone will need to be shaped a little because the well-hole itself is a tiny bit out of round and it will not drop in, so I am going to sand a little off the lip to compensate. Thanks again for all the posters for sharing their knowledge.

Hey I had similar situation...Dobro shelf edge...but I was working fora company....we had 6000 volt laser. I had a 1/6 inch thick disc, laser cut to
the width of the sound ring shelf. The ring rested on the sound ring...the cone sat on top. I also thought about using brass "shim stock" maybe .005 of an inch thick...and place them in 8 locations on the sound ring, where the spider
leg would be contacting the sound ring.
I was personal friends with Randy Getz...who was like a brother to Bobby Wolf.
He told me Dobro used to glue the cone to some of the sound ring shelf, so when the Dobro might have been bounced around during shipping so the cone contact edge wouldn't get messed up with the fit to the sound ring, As we all know when you ship a guitar it makes sense to relax the string tension...but the Dobro you don't want to release to a point where saddle loses contact with each string. Same applies when you change strings...replace one at a time. Let me know if you don't agree...with any of this.
Always looking for new friends. If you're looking for a terrific luthier...contact Jack Stepick.
Be Well
Tom

Dobro(tm) fact is stranger than fiction.

Speaking from first hand experience and observations on approx 3,000 spider bridge resonators (yeah..Paul and I did a rough calculation. scary, ain't it?)

Pre-war and OMI Dobro(tm) construction quality was, uh...... "variable".

Take guitars of consecutive serial numbers and they might well be vastly different in terms of construction & setup quality. So, why would s/n 1 and s/n 2 be so different? Why would cones be glued in on some, occasional felt and/or tacked in with brads on others and simply resting on the shelf on the vast majority?

We surmise that once a guitar was built that instead of going back and fix some egregious issue (and there were many), the shop just decided to use the heavy handed approach and "cure" whatever issue presented itself with felt, nails, glue, whatever. It just took much less time and who was to know?

Why do we think that? It's because we've been inside of these guitars and correctly addressed the problems at hand, typically with the cone ledge and/or top. We've ripped out glued cones. Pulled nails. Rebuilt and re-routed shelves and taken the time (and $$) to do proper restorations and setups.

The other issue which has been common over the years is that when guitar owners experience issues over time with pre-war or OMI built guitars they often incorporate DIY hacks to remedy issues. Somewhat understandable because of the paucity of reso-trained luthiers/repair folks. I have personally observed a significant number of guitars where you have to deal with a DYI hack on top of the original factory hack. Not pretty.

I can't tell you the number of times at ResoSummit where my primary task was "triage"/disassembly/reassembly and I'd rip apart a Dobro(tm), look inside, step back and blurt out... "WTH am I looking at?"

More often then not, some kind of kluge.

Thankfully, contemporary builders will usually build to a much higher standard.

Ahhhh..The Dobro(tm) repair, restoration business.

There are stories...

h

Tom Engleman, I don't recall ever hearing the term "sound ring" in the past for what you're describing.

I guess you're referring to the narrow wooden ledge that contacts the edge of the cone? 

Beard refers to the screenless rings they sell on Resophonic Outfitters as the "sound ring."

The cone in my '33 Regal was actually nailed in, very surprising.