I've not checked the pre-filter yet! I was thinking that I needed a torx screwdriver to access it, but have just worked out that I don't!! So I shall do that now and will make a New Year's resolution to wash this on a monthly basis. I've not used it for plaster dust but it has been used for something similar ( ash from a wood burning stove). Oh dear I feel another resolution coming on! Another thing its probably not very happy about is the amount of builders gritty sand that is being tracked into the house. Its like grit so I'm guessing thats not doing the machine any favours.