This is my mundane trade. I am a painter that specializes in traditional painting techniques. Gesso is a sealer primes that is used on rigid surfaces only. Any flexing of it will result in severe cracking. Gesso is made from hide glue and powdered gypsum (slaked plaster, commercial whiting is the contemporary substitute). Rabbit skin glue is the most favoured of the hide glues because it is the strongest. The best rabbit skin glue is purchased in sheets and pulverized with a hammer until there are very small chunks. To make a very strong glue for woodworking, place 5 oz. (by weight) into one quart of pure water and let it soak overnight until the chunks expand, become whitish in color and then it is ready to heat. Glue should be heated in a double boiler taking care that the glue does not boil. You heat it until the glue dissolves entirely. It you want to use glue that will be better on a more flexible surface, then 2 1/2 oz. per quart of water is used. Once cool the glue mixture turns into a very rigid gel and can be kept for a few days in refrigeration, after that it will start to liquify and spoil.
Traditional varnishes were oleo-resinous varnishes which were made from a combination of drying oil (linseed, walnut or poppy oil) and resin (damar, mastic, copal or amber). These are products that can be purchased readily at a good art supply store. Amber is the hardest of the resins and mastic is the softest. Mastic is common among the Greeks and is used as a gum called Mastika and is available in Greek grocers (or art supply stores). If you are looking for very high quality raw artist's pigments and resins, google Kremers. They have a store here in NYC downtown where I purchase my genuine ultramarine blue (Lapis Lazuli), my resins and my pigments.
Damar is readily available and is made in its thickest state in a 5 pound cut (that's 5 pounds of resin to a gallon of turpentine) for your purposes you can certainly scale it down to 1 pound to 1.25 quarts. You make up a cheesecloth sack that the resin is placed in and it is suspended and tied to the top of a container of turpentine so that the sack doesn't touch the bottom. Then you wait until the crystals have dissolved. You can mix this together with some Venice Turpentine and linseed oil (i/3of each) and you will have a typical final varnish that was used as a final varnish on thoroughly dried egg tempera paintings and occassionally wood work. It is very sticky like tung oil but very thick. The addition of Venice Turpentine (an art supply item made from the pitch of pine) causes the fluid to act like an enamel and smoothes out brushstrokes. You must be patient though because it doesn't dry quickly. The addition of a few drops of japan drier or cobalt drier will make it dry quicker, but these aren't traditional driers. To be completely traditional you'd add copper arsenate or litharge of lead (both toxic, as is Japan and cobalt driers) so don't ever use it on anything that will go in your mouth (like wooden plates and goblets).
If you want more information on what was used in period, it's not hard to find. There is a manuscript about painting materials and varnishes and finishes for artisans that was writen in the mid 14th century that has been translated into modern English and it's called "The Strassbourgh Manuscript", a hundred or so years later there is "Il Libro del Arte" by Ceninno Ceninni which was written at the time of the great Italian masters of the 15th century (Ceninni knew most of them).
There ya go.

I did not use a dryer. I was trying to create the varnish based off the listing of compounds found in a find in Denmark. I can't remember the name of the report off the top of my head. I found it on JTOR a couple years ago and just wrote down what was used on the shield. I started with that information and looked for how to use them to create a varnish. Leadoxide was one of the compounds in the varnish. I am thinking they could have used that as the drying agent.

I guess I will fudge it and try the Japan Dryer.

Thanks. This has been of great help.