The good folks from Cardinal (a finish manufacturer making really good products) just turned me on to System 3 epoxies. They make one formula that is specifically used as a sealer/coating and has very, very low amine blush issues. I'm about to try it, and I'll report in.
The amine blush issue is a big deal with epoxies. It can prevent subsequent coats of finish from adhering well, and it also acts as an anti-catalyst for polyesters. You can deal with it by washing the amines off of cured epoxy with water...yes, water, not any kind of chemical solvent. I've been doing that with WEST epoxy used as a sealer...light sand, scuff with ScotchBrite, and then do a wash with a damp paper towel.
I have had really good results pre-sealing and pore filling with two coats of epoxy. It does a pretty good job of filling pores, and the great thing is that once cured, there's no further shrinkage. It' NOT the traditional dyed pore filler look as per Martin, but I'm not trying to do that.
Over the epoxy I'm using either Simtec's "Adhesion Promoter" or now the Cardinal equivalent urethane. These are "isolator" coats...sealers...particularly needed over rosewood or cedar, both of which have oils that are also "anti-catalysts" for the polyesters I like as build and top coats. They are also superior sealers under nitro, as Addam Stark is finding out.
The real key here is getting great adhesion, and sometimes you have to graduate from one material to another to another to get this. It's like the very best chrome plating over steel. It has to be copper, then nickel, then chrome to really get fantastic grip and the great look.
For those who want to try varnish, take a look at the Epifanes line. They have a formula that does not need to be sanded between coats as do normal spar varnishes. Behlen's Rock Hard is also favored by some American luthiers. I see no reason why these varnishes would not work really well over epoxy, and the finish could be brushed on in a simple clean room.
Another nice look is Waterlox tung oil (really a long oil varnish) directly over epoxy sealer/pore filler. It's not the music store finish glossy finish that so many expect, but it's a really superior take on an "oil" finish.
Yes, I'm a big fan of epoxy as the basic sealer. It adds a level of moisture resistance that no other coating can, it works nicely over a lot of stains (Behlen's SolarLux or US Cellulose MEK soluble dyes), and I do believe that it makes for the best adhesion of subsequent coats of anything that you can do. The thin epoxies also have fantastic wetting out properties and will really pop grain figure and natural wood colors. They'll also find any hidden flaws in the wood and help keep it together.
A number of American luthiers do a whole inside and outside treatment on Brazilian rosewood stump wood which can be problematic using epoxy on the joined but raw parts and then run the wood through a drum or wide belt sander. The epoxy penetrates well and helps to stabilize the wood. Look inside the guitar, and you cannot tell that this was done. The Smith CPES epoxy is practically water thin and is great for this.
I also know that a number of high end classical guitar makers are epoxy sealing guitars under French polish...and they aren't talking about it. It cuts down the FP time by at least 50% because all the basic wood prep and pore filling is done and stable in two days, and the FP can be less than half the number of coats.
Another more traditional technique comes from Romanillos who uses egg whites as a ground (sealer) particularly on tops underneath FP.