You are right -- fillets are not acceptable in "Maximum Preservation" projects, according to the Fine Art Care & Treatment Standards (FACTS) Guideline (revised Jan.2000).
That doesn't mean we should never use fillets, it just means we take an exception to the "Maximum Preservation Standard" when we do. The same is true for many popular design features, and when we use a wood frame without a special lining in its rabbet.
FACTS recommends that we list exceptions to the standard, as a way to uniformly describe the level of preservation for each project. They even provide special write-on dustcover labels for the purpose.
The main problem with fillets (and unlined rabbets) is outgassing. For designs where a fillet is to be used, we can lessen the problem by applying a gas-impermeable barrier, such as glass or metal, which would seal the unfinished bottom of the fillet. It's not a perfect solution, but the best protection for use of fillets in preservation framing.
By the way, rag board -- no matter how thick -- is not a gas impermeable barrier. Neither is Mylar. These and most other materials should be considered buffers or filters, in that they give a limited amount of protection for a limited amount of time. They are not permanent, though, because contaminating gasses can eventually saturate, penetrate or pass through them.
The gasses are bad news because they often chemically-react with art media, which might cause fading, color shift, disintegration or other nasty stuff.
Gas Impermeable barriers are glass or metal, for practical purposes in framing. Also, "Camger Varnish" is said to make a gas-impermeable barrier, which will last as long as the varnish does. But it's a painted-on coating, and would eventually crack. How long is that? Who knows? Probably many years.
Suppliers of archival materials offer metal (aluminum) tape which has a good adhesive and makes a good long-term barrier.
Stick it on the bottom of the fillet and mat, and then use a rag board liner to prevent abrasion of the art by the metal tape.
For a lot of projects, all of this is academic. But for those jobs that need the best-possible protection, these are a few of the ways we can provide it.
One more thought...don't hold your breath waiting for Jeff T. to reveal his fillet-installation technique online. He has a class for that, which is how he makes his living. I haven't attended that class yet, but I've heard it's a good one.
See you at the shows...
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Jim Miller,holder of CPFcm designation; FACTS/GAFP Committee Member