When Is Cleaning A Coin So Its Not Lost Forever Better Than Total Loss?

Cleaning a coin properly is by no means a "no no". It's perfectly OK. Emphasis on the word "properly".

On the other hand, improperly cleaning a coin will destroy its numismatic value (if it had some before).

There are two aspects to understand:
1. Does the coin you want to clean have any numismatic value? If not it doesn't really matter..
2. What are the techniques you can use to clean and what it actually means to clean/perform restoration.

I advise to search the forum for topics on cleaning coins, it's a common topic and we have a lot of discussions on it.

That's well said.



to the CCF!

To CCF! Hfjacinto said it well. I agree.

Thanks all. Didn't want to destroy any like I have seen a few done

The general advice we always give people is "don't clean coins". The "don't" should have a little asterisk next to it with reference to a list of exceptions and disclaimers, but we usually don't want to confuse the newbies with unnecessary edge-case scenarios, when the general rule is far more often than not going to be true.

The generally accepted exceptions to the "don't clean coins" rule are:
- Ancient coins. All ancient coins currently in colelctions were dug up from the ground some time in the last 500 years or so. Most of them looked like little green rocks when they were freshly dug up. All of them - with the likely exception of gold coins - needed cleaning. Exactly when to stop "cleaning" an ancient coin is somewhat debated, but simply in order to identify it properly, it needs some degree of cleaning.
- Metal detecting finds. As with ancients, they are covered in literal, physical dirt when they are dug up and it needs to be washed off. Adverse soil conditions may require more aggressive cleaning methods.
- Coins stored in old PVC plastic album pages. Plasticized PVC or "vinyl" slowly emits sulfuric acid as it ages and degrades; this acidic goo sticks to coins and eats away at the surface. The goo should be removed with acetone.
- Coins covered in oil, varnish, lacquer, paint and stickytape residue. This is regarded as "foreign matter" as it is not chemically bound tot he coin's surface. Acetone gets rid of most such debris.
- Coins with active corrosion. Precious-metal coins don;t suffer from this, but bronze disease, iron rust and aluminium cancer will expand and grow in an almost-biological fashion if they are left untreated and the conditions are right for continued growth. Such forms of corrosion must be removed, or there will soon be nothing left of the coin but a crumbling mess. This, I suspect, is what the OP refers to when they mention treating coins so as to be "better than a total loss".
- War medals. Nothing can stop an old soldier from wanting to polish up their medals, so we don't even try. Unlike coins or most other numismatic items, war medals generally aren't badly punished on the collectors market for being cleaned.

Note: saving a badly corroded coin will almost certainly result in a "cleaned coin" of minimal value. But in at least some circumstances, a cleaned coin is better than no coin at all.