I am still kind of a beginner in the music field; I'm learning classical guitar by myself, and have no one that I can consult or ask locally!
So I'm asking if cleaning Classical Guitars (the strings and the body) by rubbing alcohol has any negative effect in the long time - may that damage/ruin the strings for example or something like that?
You're really asking about two different things, strings and the guitar body. Another area of hot topic is also the fretboard.
For a classical guitar, you generally have these made from two materials: nylon (or some other synthetic or semi-synthetic material) and steel windings. The life of these treble/bass strings are totally dependent on use. Your perspiration chemistry will determine how well these hold up to standard use and abuse. But in general many players use a dry microfiber cloth to wipe the strings off after use. This usually helps most people with sustaining the life of the average string. For those of us (myself including) with more damaging effects on strings; treated wipes or applications can be used too. It's important to remember that anything you put on the string shouldn't be put anywhere else on the guitar though; as the testing and "guarantee" of such item (if it even exists) will only apply to the strings themselves.
Like the strings mentioned above, bodies can consist of many different woods and even more finish types which all will have different reactions to solvents used for cleaning on them. In general; most players and manufacturers will agree that using rubbing alcohol on a finish like the body (or top, back, sides, neck, etc.) is a bit too harsh of a cleaner. Depending on the finish type; the alcohol could (worse case) eat away or wear down the finish, or even cause it to "cloud" over and haze. Most guitar owners clean their bodies with dry, lint-free cloths, and many manufacturers suggest this same method. Like Taylor, Alvarez (expand "How Should I Clean My Guitar"), and Martin. Some manufacturers recommend a polish (as in a few examples above) but note their suggestion of a clean, dry cloth for cleaning the body.
You didn't mention the fretboard but I can assume that it may come up, especially if you happen to get some rubbing alcohol on the strings (and potentially the fretboard). Rubbing alcohol tends to have a drying property and the same can apply to untreated/finished wood, which generally is the most common type of fretboard on a classical guitar. It's important not to get anything on the fretboard as most manufacturers will not recommend it.
Continuous application of rubbing alcohol to an unfinished fretboard will more than likely cause it to visually look dried out, and even may cause the wood to crack inside the grain.
If you are going to use rubbing alcohol use the suggestion that most cleaners have on the back of bottles: use it in an inconspicuous place to see if there is any sort of reaction.
But best bet is to go with something specialized from the manufacturer, or just a dry lint-free cloth to wipe away smudges and grime (if it's really bad, dampen the cloth ever-so-slightly, but be sure the finish doesn't sit with water on it).
I personally would never recommend this.
It might be OK for a typical mass produced modern instrument with a thick poly finish, but even then I don't think I would want it finding its way through minor finish cracks and getting into the timber.
More seriously I'm sure it would be bad for a higher end shellac/French polished classical. (Most of us don't have one of those yet though!)
Also it may affect the rosewood or ebony in the fingerboard - I suspect it would dry out the oils in such timbers leading to shrinkage or cracking, unless again it's under a thick poly-type finish.
(Someone else might advise if they have used it on the fingerboard and used lemon oil or similar to restore the oils afterwards. I personally use nothing but simple dry cloths for cleaning up the inevitable dust or finger oils, plus a bit of breath if clearing stubborn dirt.)
It's not very common, but some classical guitars (and maybe acoustic as well?) have shellac finish. Alcohol dissolves shellac, and you should keep it away from such instrument. Even alcohol based deodorant on your clothes can damage it.
I would never use alcohol on a fretboard or bridge. The natural oils in the wood, if kept humidified, are all it takes.
I've never done that:)
There is something peculiar I found. Brasso (https://en.wikipedia.org/wiki/Brasso). When the guitar had grime on it, Brasso really cleans it well, plus it doesn't dampen the body of the guitar.
I always use Brasso for cleaning my guitar. Hope this helps!
