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 (cf the (1)) and others would require to ask questions in the stack chemistry (cf the (2))
First: our current technological level isn’t advanced enough to solve that issue. The solutions proposed below are more workaround than anything else.
As mold needs moisture, cellulose (it eats it) and air to survive, the most efficient way remains to control humidity (which is unfortunate for those who can’t do it).
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) isn’t enough as the molds generate countless tiny spores (their “seeds”). A single spore can start by itself a whole new mold invasion. The spores are quite tough to destroy (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 a next invasion:
—by making the environment harsh for the molds to grow
—by deactivating/killing the spore
Killing the visible mold: it’s easy to do it with bleach, peroxide … (cf the below).
Killing the molds deeply rooted: this 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 (there is probably more effective one) (1).
1/Preventing (most of?) further invasions:
—By making the environment harsh for the molds to grow: molds seem to be sensitive to pH. Some will be destroyed by an acid pH, but (most) others will be destroyed by the very basic pH (over 9).
—By deactivating/killing the spores: that’s the really difficult part.
Increase the pH:
It seems that a basic pH can kill the mold and (and maybe deactivate the spores?). That’s why is used.
Borax (Sodium borate, sodium tetra borate, or disodium tetra borate) is quite basic, so it will Kills/prevent or slow mold growth. (Most, but not all types of molds needs 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 European Commission as a potential threat (fertility issue, among others).
As borax become 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 per carbonate: (2)
—Trisodium phosphate (TSP): (called “borax substitute”). Does it work despite its biodegradability? (2)
—The sealing solution (seems to be the most effective):
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)?
2/Killing 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 in 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 ever.
Basic solution (baking soda, borax…) might kill molds that need acid ever.
What work (a little) for me: mix some hot water with borax, then add 30-50% of bleach, and some dishwashing soap. Remove the mold with a sponge.
3/Killing/deactivating the spore:
Since spores can float in the air, some recommended using Chlorine Dioxide: it’s a gas which kills (deactivates) the spores (of most species). 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 chloride". I’m still looking for that…). 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 hospital uses it.
In brief, many questions remain. We are still waiting for THE scientific invention to deal with that (a gene therapy?)
(3) The PU coating DIY technic is easy: just missed some PU expansive foam with acetone to get PU a water-like substance that could PU coat stuff (11 gr/100 ml works well for camping gears too, it’s not hydrophilic and waterproof, but ‘breathable’). But watch for the isocyanate: it can penetrate your skin: latex gloves should probably OK for 10 minutes, but no more (or use a PP plastic bag as gloves).