Now there’s a job description you don’t hear every day: violin forger. 
I have a nice violin - not a strad by any means - but I’ve been told that it’s better than average. It was given to me by my parents when they were cleaning out their house, and as far as I know, it has not been played in 30+ years.
I’d very much like to get it out and sell it to someone who can enjoy it. So, how does one go about refurbishing a neglected instrument?
Find an expert. It’s easy to say, and your location field doesn’t give a great amount away 
Seriously, the only way of getting a realistic assessment of an instrument is for someone to see it first hand. If there’s no obvious route to do this where you are, search out any local violin teachers or other musicians, and find recommendations of where to take it. If you can get a good recommendation, you needn’t worry about shopping around to get several opinions. However, if you’re stuck with using the yellow pages of the nearest big city, then it might be worth (very subtely) getting two or three opinions. It’s a small enough world that the vast majority are honest, but there’s a few charlatans out there.
Do you mean that you want to clean it up so it looks nice to potentally buyers? You’re not talking about, like, refinishing it, right? 'Cause that would be bad. Most people want the original finish and patina on the instrument.
Here’s how I would clean it, if it was brought to me at work:
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Get a small, clean paintbrush, and open the vents on your vaccuum cleaner hose to reduce the suction. Use the paintbrush to flick out the dust in the crevices while holding the vaccuum nozzle nearby to catch the dust.
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Slightly dampen a soft, clean rag. You can use water-- the standard for how wet to make the cloth my curator always told me is “damp as a dog’s nose.” If the dirt is nasty or greasy, you can use mineral spirits to dampen the cloth. (Mineral spirits are good for cleaning wood because it leaves harmless residue and it dries very quickly. At my museum, we use Sunnyside brand.) Only wipe the wood if it’s not chipping or flaking. If the wood doesn’t dry in a few moments, you’re using too much liquid.
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If the wood around them is smooth, with no snags, you can use a dampened Q-tip to clean in those hard-to reach areas.
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Use the low-suction hose and the paintbrush to clean out the inside of the case. If the exterior of the case is leather, only use the paintbrush to clean it.
NOOOOO!!! Not unless you want to remove the spirit-based varnish!!!
I strongly recommend doing nothing to it at all by yourself. It’s far too easy to do damage if you don’t know exactly what you’re doing.
Interestingly, I can reveal that we once found both a genuine Stradivarius and an undiscovered Rembrandt in my grandfather’s loft.
Unfortunately Stradivarius couldn’t paint for toffee and Rembrandt made lousy fiddles. 
Perhaps you know more about this than I do, because I’m certainly not an expert in violins, but I’ve used mineral spirits on all sorts of wood finishes and never had any damage to the finish. I used it last week to clean a lovely dulcimer from the 1830s.
Sorry, yes, I was thinking of methylated spirit. But I stand by the assertion to not do anything at all, and get a first-hand expert opinion.
With all due respect to the care that Lissa undoubtedly took with the dulcimer and other instruments, it is usually a fool’s journey to try to clean and restore a very valuable instrument without serious formal training.
If you think you have an instrument of moderate value ( over $ 10,000.00 USD ), I suggest you get a copy of The Strad. It is the world-wide monthly publication specializing in all things violin and string. Sprinkled throughout, and certainly in the back, are the names of several dozen reputable makers and restorers of string instruments.
Please get a copy, or pay to read it online, and glean from that a list of folks to contact. All but the snootiest and most pricey and elite will be glad to at least examine the instrument for nothing, or a very small fee and give you a detailed estimate on cleaning, repairs, re-glueing or tightening up of the body parts before they do a minute’s work.
It would be worth it to invest in a quality estimate, for surely it will A) protect or increase the value of the instrument, B) make it look and sound nicer ! and C) provide you with a current estimate of it’s true market value for insurance purposes. Any dealer will also be willing to give a formal Appraisal. If it is of moderate or greater value, you might want to have the Appraisal done so you can have it insured.
My two cents. We don’t own a spectacular instrument yet but daughter ( aged 14.5 ) is showing serious promise. It is likely that if she does pursue a career, during her college age we will try to purchase one of the higher end instruments being made, such as the excellent Samuel Zygmuntowicz instruments.
The violin she plays now is valued at a point where we do have it insured.
I Previewed that last post, but I’m feeling badly about it anyway. My point is suggesting professional intervention was to preserve a potentially valuable instrument in the financial sense.
I am sure that in tending her instruments, Lissa has taken great care with their cleaning and restoration and hasn’t done a thing to mar them or diminish their music-making capacity. I’m sorry if my post came off in a negative way. My only goal was to encourage a certain treatment of a certain grade of instrument.
Is the dulcimer the instrument where you have to make two sounds at once? A background strumming and then the melody ?
No offense taken. You are right in saying that it’s better to be safe than sorry.
All I do is clean artifacts-- I’ve never done any restoration work. (My museum is opposed to it.)
As for the dulcimer, I have no clue how it is played. It’s a large, heavy, rectangular box with upwards of forty strings. (I counted them for the paperwork, but can’t remember.) This particular instrument was absolutely lovely, made of burled maple with delicately carved sound-holes. (Are they called “f” holes on a duclimer? I don’t know.)
That sounds like it’s a hammer dulcimer - played with hammers, unsurprisingly, which are small light items, about the size of a spoon. You can hear one in the sample on this page.
As Lissa has answered, I’ll chime in to say you’re almost certainly thinking of a Zither, although the dulcimer is a member of the same family.
Finding a Stradivarius on ebay - you never know! A woman named Nadia Tupica found one lying on the side of the freeway in Los Angeles.
(Los Angeles Times)