I think that the advice that Ron gives in conjunction with the FACTS guidelines should be discussed with your client before continuing with the addition of a fillet.
If the fillet is to be joined to the opening of the frame, you didn't specify if you are using any mats, then I would consider section 10.02 of the FACTS guidelines which states:
"10.02 All artwork shall be a minimum of 1" (25 mm) away from wood surfaces, including wood frames, fillets and liners."
That is easy to accomplish if you are using a matboard as a spacer between the print and the glass AND you are not using a fillet on the matboard itself.
If you want to use a fillet on the matboard, then there is a rather shady area to contend with:
"5.05 Artwork shall be completely separated and isolated from any potentially damaging materials by an impermeable barrier such as glass or metal."
This could be accomplished by using the metallized tape such as Erin mentioned. But, if you take both articles at their <u>printed face value</u>, there seems to be a conflict in that 10.02 says there shall be a "minimum of 1" away from wood surfaces such as frames, fillets, and liners". This, in the purest sense of the article, is impossible unless you build the matboard up from behind with no less than 1" of spacers above the surface of the artwork. The proximity of the wood to the artwork in both articles is the subject of contention, in my opinion.
Now, I admit, it is a nitpick and there isn't a logical method to follow to adhere to both articles of that portion of FACTS. But, I also admit that, to many framers, the FACTS standards are taken to be "guidelines" and not final rules to follow under all conditions. That has been hashed around in another recent thread.
So I would discuss it with your client and the two of you come to a conclusion as to how far into the "preservation" aspect of the framing the client wishes you to go. I personally feel that, for most normal treatments, the barrier tape seems to be the logical step to use to protect the surface of the artwork from acid migration from the fillet. The effort by you to protect the client's artwork is backed by guidelines that are proven and have been checked out for long term applications.
It is much better than pressing the raw wood surface of the fillet directly against the surface of the artwork, in my opinion. One other thought, if you want to further ensure that there is an adequate barrier against acid migration, you may want to seal the back side of the fillet prior to installing it with a varnish or other clear finish.
You didn't mention the mounting of the print, but if you encapsulate it to the ragboard backing, the clear film would provide a barrier from the fillet. Of course there would be some reflection issues, but they may be overcome by the desire to have a fillet as part of the look.
If you've ever seen and old print that had a fillet against it in the frame and seen the burns left on the art - you would understand why the FACTS standards recommend against fillets against the art. The foil tape on the back of the fillet will certainly help but it would still leave a portion of the un protected fillet within an inch of the art.
Rag board is not a barrier! Even though "They" have been telling us that for years, FACTS research found examples of acid burns from a regular mat right through the rag board "barrier"onto the face of the print.
The one inch rule mainly is referring to the distance from the frame to the edge of a print. FACTS research found examples of art with acid burns around the edges from the resins in the wood frame. So seal the frame with foil tape if the art comes closer than one inch.
Better safe than sorry. Show your customer the FACTS standards and let them see for themselves.
Maybe do french lines instead of fillets to be on the safe side.
And with all that being said, at this time, I have 20 different fillets soaking in distilled neutral water, now for 27 days. Only 3 have tested on ph paper, (which reads 1-14 detecting acid AND base), at anything other than 7. All of the bass wood fillets have consistently tested neutral.
I'm looking for a group at one of the Universeties who would like to make it a real endevor and test different woods that are used in our industry on a regular basis these days.
As to frames that burn through mats, glass, backing, art, and the universe . . . thank gosh Southern Yellow Pine is no longer a common picture frame wood.