Back in the day on the OLF we had a member, Michael Dale Payne who is a great guy and was very generous with all of us with his time and more. Michael is the one who created the OLF plans for the OM, small jumbo, and possibly the medium jumbo too. He originally gave the plans to the OLF for distribution to benefit both membership and forum. Later these plans found their way to Stew-Mac. Hundreds of very cool guitars have been built from his plans.
Mike was our resident expert on this question and likely 50 times or more he was known to post a direct answer to David's direct question. It came up so frequently I wondered if he assigned a key to a macro that would make that post for him...
That answer was and is: Epoxy under shellac is fine, shellac over epoxy is fine but.... EDIT: epoxy over shellac should be avoided....
![[headinwall]](./images/smilies/headbangwalluf8.gif "Mad")
Regarding Mike Doolin that quote is very old, over 10 years old IIRC and this was back in the day when Mike D. was pioneering early waterborne finishes for Lutherie. Mike D. used KTM-9 back then which was sold by LMI hence the instructional writings on LMI's site. I believe that Mike D. moved away from KTM-9 a long time ago.
Mike was keen to only use System III back then and his comment was specific to System III as per Roger's comment.
The only thing to add is that lots of folks, including Mike Doolin would apply a very thin wipe-on coat of thinned epoxy over the entire surface of the instrument's woods that required pore filling (not the top....). The reason for this wipe coat was to even out the color, pop the figure (epoxy pops figure line nobody's business) and you can see from Mike D. quote above he mentions epoxy's figure popping qualities as well.
Many of my guitars have this wipe on epoxy coat too for evening out the color and popping the figure.
So..... if you are hankering to pore fill with epoxy which can work very well by the way and you are keen to pop the figure on your very cool back and side set, neck, etc. with a final, thinned wipe-on coat this coat acts as a sealer as well.
There has been some disagreement about this in the past on the OLF where some believed that if the entire surface has epoxy on it shellac is redundant where some felt like a sealer is necessary. Of the 5 of so of mine that I kept all of them have epoxy pore filler, a color wipe-on coat of epoxy, and then the final finish applied directly over the epoxy - no shellac. All instruments are trouble free and look like new today except for fret wear....
I have french polished with shellac several to date with the shellac directly over the epoxy and these instruments have held up fine except for my personal favorite guitar a 2.9lb hog/euro L-OO that is shellac over epoxy and on this one I have worn some of the shellac off the back of the neck and like it this way. I'm on my way to having a Willie Nelson Trigger of my own.
So the real answer here is it depends. If using epoxy to pore fill and not covering the entire surface only leaving epoxy in the pores a sealer coat is a good idea. If using an epoxy wipe coat many, not all, believe that a seal coat is not necessary. Those that I know who used a seal coat over a wipe coat of epoxy did not use shellac and favored a vinyl sealer and reported success.
