I just find it useful to recap all the information I could find on the web about it and the unanswered questions and hypotheses that still need to be tested. Please feel free to correct and update my answer or by commenting below (Verified information and test result are more than welcome).
This answer is a work in process: many things written here need confirmation (and others would require answers from the stack chemistry)
In brief: our current technological level isn’t advanced enough to solve that issue... The solutions proposed below are just a workaround. As mold needs moisture + cellulose (it eats it) + air to survive, the most efficient way remains either to control humidity (which is unfortunate for those who can’t do it), or (easier and cheaper) to simply wrap the things the mold love to be (e.g. wood, or the cellulose of a plasterboard) with a hermetic substance (you'll find below a super cheap DIY solution).
Molds are difficult to eradicate because:
It can live deeply inside stuff. So only killing the visible mold isn’t enough.
Killing all the mold (the visible ones and the one deeply rooted in stuff) isn’t enough as the molds generate countless tiny spores (their “seeds”). A single spore can start by itself a whole new mold invasion. And their spores are really tough (1) and they also float in the air.
So to get rid of mold invasion, several actions seem to be required:
- 1/ killing the visible mold,
- 2/ killing the molds deeply rooted inside the wood, wallpaper…
- 3/ preventing the next invasion by:
- making the environment harsh for the molds to grow
- deactivating/killing the spores
Killing the visible mold is easy: just use bleach or peroxide … (cf the below). But it could come back in a day or two...
Killing the molds deeply rooted is another story as you need something to help the product (which kills the molds) penetrates inside the wood. You need to use a “surfactant”, like dishwashing soap (or a better one, which one?) (1). (I never worked for me).
The easiest, cheapest way, and often the only solution (if you can't lower the humidity): use a hermetic coating
There is a DIY super cheap solution:
by mixing 10gr of polyurethan foam with 80gr of acetone you can waterproof stuff with this water-like substance. But it only works if you put several layers (at least 3). A single layer isn't hydrophilic nor waterproof, but ‘breathable’. If I remember well, each layer is pierced by some tiny bubbles and thus tiny holes. So, the more layers you add, the more waterproof (but also rigid) it becomes. With this cheap stuff I waterproof my camping gears, and humid walls. Molds can't pass through several layers. (But be careful: acetone can dissolve / destroy many stuffs like some type of plastic).If needed put a layer of paint over it at least to protect it from UV, and because, if you take the cheapest foam, it'll be UV sensitive, the coating will turn yellow, then brown after a few weeks/months (which can be quite nice).
BONUS: you can reuse the spray can for month by using this trick:
⚠ WARNING: be very careful to not touch the mix of acetone and polyurethan foam, not even with gloves as it contains isocyanates (present in the hardener or catalyst of polyurethane-based two-part paints. It will be dissolved by acetone and will easily penetrate your skin and most gloves ― I recommend applying it using a paint roller or, if you really need to touch it, using Butyl gloves, or (cheaper) 2 layers of latex gloves under some DIY gloves made out of PP plastic bags ― latex gloves will only protect you for few minutes, maybe even seconds).
other information (theorical)
UVC lamp could (theoretically) kill and prevent the invasion
They work in 2 ways:
- they destroy (some) mold with ultraviolet radiation (and their spores, which is very hard to do)
- they also destroy them with the ozone gas they are producing.
Obviously, you have to be very careful since these lamps a very dangerous for every form of life (including you). It can cause cancer, blindness... Even half a second on your eyes/skin is too much. You should simply light the lamps when you are out or wear adequate protection if you don't mind breathing ozone gas (some UVC lamps don't produce ozone though). Plastics don't like it either (eg. you monitor, laptop...) but UVC photons aren't really strong: a simple thin sheet of plastic (even transparent) might be enough to stop them (they don't pass through a transparent glass either). But don't trust me, check it yourself.
I would first clean it with bleach (or peroxide), then use a powerful UVC lamp (maybe x(?) hours per week when the moisture is high). (That should do the job until the new generation of dehumidifier/desiccant like MOF will come to the market (currently desiccant dehumidifier like this seem to be the best). Or better: an RNA treatment to either destroy them or to stop them being toxic, or even a GMO healthy fungus that would fight the unhealthy one while providing some cool benefits (green light on the dark, nice sm?). Can't wait for the future...).
How to (try to) prevent further invasions?
—By making the environment harsh for the molds to grow: molds seem to be sensitive to pH. Some molds will be destroyed by an acid pH, but (most) others will be destroyed by very high/basic pH (over 9).
—By deactivating/killing the spores: that’s the tough part.
Borax (Sodium borate, sodium tetraborate, or disodium tetraborate) has a quite basic pH (but less than bleach), so , in theory, (it didn't work for me) it should Kills/prevent or slow mold growth. Once again, most, but not all types of molds need an acidic environment.
1 tablespoon in warm water (about 40 °C—makes it easier to dissolve it).
According to many sources, Borax was effective and durable but some concerns have been raised about its effect on health (especially when ingested or when in contact with eyes). If I understood well, it was widely used for everything and anything without any concern about its potential toxicity (it’s also not biodegradable). So in results, it has been “classified” by the European Commission as a potential threat (fertility issue, among others).
As borax becomes difficult to find (in the EU) I was wondering if any strong basic solution could do the job. What about for instance a solution containing (cheap) dishwasher powder (2)? “Borax Substitute”: sodium sesquicarbonate is much easier to find (in the EU) than Borax but is it really as efficient? And what about its durability (unlike borax, it’s biodegradable) (2)?
—Concrobium Mold Control: this product also plays on the pH to remove molds. It contains sodium bicarbonate (baking soda, pH=9), sodium carbonate (washing soda, pH=12), and trisodium phosphate (TSP, pH=12). Is TSP use only to increase the pH or for other reasons (2)?
—Sodium Bicarbonate: its pH is lower than Borax so it seems to be less effective (in my case, this solution did slow a bit the process, but not much). But it’s safe.
—Sodium percarbonate: (2)
—Trisodium phosphate (TSP): (called “borax substitute”). Does it work despite its biodegradability? (2)
—The sealing solution (seems to be the most effective with the UVC):
Clean up the mold from the support, wait long enough to dry it, and then “vitrify” the support (using for instance any transparent basic flooring product). The mold won’t come back on it since a thin layer of “plastic” protect its food (cellulose) against it. As for the mold that has been sealed under the layer, it’s not clear what will happen. What would happen to a mold simply recovered with a thin layer of PU or wax? But it will protect from the spores which are the main health issue. (2)?
How to Kill the Mold?
Bleach: kill the visible mold (how? (2)), but the chlorine doesn’t penetrate into the wood, wallpaper, so it doesn’t kill the molds deeply rooted. The water contained in the bleach will penetrate the wood so that will help the ‘inside’ molds to grow faster.
Hydrogen peroxide: same as bleach, but safer (no toxic gas)
By changing the pH:
Acidic solution (Vinegar…): might kill the mold that needs basic pH.
Basic solution (baking soda, borax…) might kill molds that need acid pH.
You can try to remove the mold with a sponge mix (cold) water and bleach (50% each) to kill the visible part. Then to dissolve anything basic (like borax, if you don't want to have kids) and some dishwashing soap. Wear a mask to protect yourself from the spores.
How to Kill/deactivate the spores:
Since spores can float in the air, some people recommend using Chlorine Dioxide: it’s a gas that kills (deactivates) the spores (of most species). But... the gas is unstable and explosive from a certain concentration, so you won’t find it in your supermarket. But it’s "easy" to make ("providing you find sodium chlorite"… I didn't). You only need to mix a drop of sodium chlorite with a drop from any acid (e.i: citric acid). When the two drops meet, the gas is produced. It’s best to evacuate the building during the process (they recommend 48h). (You can find on eBay devices measuring the concentration of this gas.) Some hospitals use this technic to disinfect. (UVC/ozone lamps seem easier, safer).
In brief, many questions remain. We are still waiting for the scientific invention that will deal with that (a smart virus maybe?). Meanwhile, MOFs based dehumidifier/desiccant should be soon available. I recommend putting a hermetic layer 1) to protect yourself from the toxicity of the mold and 2) to protect the thing it's growing into from humidity. (Cf. my super cheap solution above).
Using UVC: https://www.epa.gov/indoor-air-quality-iaq/does-ultraviolet-uv-radiation-uv-lamps-kill-mold-0