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Im nearing the finishing stage of #1 which is an Appalachian dulcimer with black walnut back and sides, and sitka spruce top. i would like to do a basic brush on finish, like an oil varnish. should i use a pore filler for the walnut? any suggestions on finishes, finishing techniques, and whatever else may help would be much appreciated!

Huge subject, much discussed on this forum - try searching is one way to learn more.
For your first, and I am no expert by any means, my suggestion is maybe keep it simple.
Why do you want to fill the pores? It's not neccessary, unless you want a smooth finish. Open pores are liked by some.
Several light coats of silk finish over open pores can give a nice finish.
True-oil is easy and quick to apply, used by a lot of us ameteurs, more than would care to admit.
Padma sometimes used water based varnishes from home depot stores for filler, and I suspect, finish too - my first was finished with water based floor varnish - why not, it's tough and flexible, dries quick and sand easy. (I used Pore-O-Pack filler with shellac over it for adhesion to top coats.)
Or, if you want a glossy even coat, a simple filler - egg white, sand when applying (240 grit), rubs the dust with it into the pores and fills them, let dry, then sand level.
Repeat to your satisfaction until all filled and level, apply coat or two of shellac, sanding lightly with finer grit. (400)
Oil varnish? - Ace (just one that stuck in my mind) make an oil varnish I've heard several recommending, again there are many others in old posts.
Applying - brush fine, but use a good one, one specifically for lacquer. Follow maker's instructions and you can't go far wrong, sand ()with 400 between coats for adhesion, when nearing last coats, sand maybe 400->600->1000->1200 grits, thin varnish last coat to apply.
Polish up with Meguire's (automotive polish)
Maybe someone with more experience than me will pitch in, even if only to correct my mistakes!!
I'm just bumping the post up for you.

Enjoy your instrument, let's see it on the forum when you finish.
Good Luck!

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The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.

thanks colin for your informative post. i thought my thread was going to die with no replies. i didnt think to do a search on the subject.

my pro luthier friend suggested MinWax 209 (natural finish), which is an oil finish you can get at any hardware store. do as many coats as i desire and buff it out with 0000 steel wool. i decided to leave the pores open to get a rustic look (its a mountain dulcimer!). Do I need to prep the spruce top, since it's softer?

any suggestions on what to use to fill gaps? i have a few 1/64" or so gaps, where the heastock meets the sides, heelblock meets sides and fretboard. I was thinking Titebond mixed with wood dust. Is this a good idea?

As long as it's well (finely) sanded spruce should be good to go.
For gaps Titebond, (thinned about 10 - 20% with water) can be used with wood dust to fill larger gaps, but the colour darkens. Use a syringe to try to inject the mixture into the gaps, otherwise it may sit at the surface of them rather than fill the gap.
Another thing is superglue, - I just fill the gaps with wood dust, and then apply thin superglue which wicks into the gap (again it darkens the wood dust) But with Walnut you will hardly notice it.
But don't do this on spruce or beside it without sealing the spruce first with shellac , maybe 2 coats worked in the gap - otherwise the superglue (or even titebond ) will soak into the spruce and turn it yellow, especially the superglue. It's ideal for fine gaps you couldn't realistically get the titebond mixture into.
MinWax should be good, I've heard of many amateur instruments finished using it. I haven't tried it, but it's just I can't get it in UK.

_________________
The name catgut is confusing. There are two explanations for the mix up.

Catgut is an abbreviation of the word cattle gut. Gut strings are made from sheep or goat intestines, in the past even from horse, mule or donkey intestines.

Otherwise it could be from the word kitgut or kitstring. Kit meant fiddle, not kitten.