Surplus masks can work if they are still in good shape and seal up well, if taken care of they can last basically forever. Downside is that they and the filters for them are relatively expensive and you aren't going to find one in a shop, it'll pretty much have to be over the internet.>how useeful are those masks for spray painters and how long do they last?
If you are talking about ones like pic related, they work just fine with the right filters. You want at least P100/P3 rated filters or equivalents thereof, I am not sure if the naming standard is the same in Germany.>what if the filter gets wet?
They'll stop working, unless you get a filter with the plastic case around it which is better anyway because they can be disinfected. If the paper in the filter gets wet, go ahead and trash it.>how would you properly clean and dispose of it after having been outside?
Spray/wipe it down thoroughly with 70 per cent or higher alcohol before taking it off, make sure to wash your hands really well, then take it off without touching your hands to your face. If you are using exposed filters, soak them in alcohol then take them off and toss them out, if you are using the ones with plastic cases then they can be left on after the outside has been disinfected, the virus that is trapped in the filter will remain trapped in the filter.
If you are talking about disposable dust masks then get ones that are N95 rated and tape them to your face around the edges to ensure a good seal and follow the same decontamination procedure outlined for disposable filters when you are taking them off.
Honestly if it gets really bad I'd suggest holing up and not going out unless you really have to, I'd get a mask and one or two spare sets of filters, and spend the rest on food and water.>>1701634