Oh, also--if you have some cartridges for one type of respirator facepiece and they don't fit another that fits you better, there are adaptors you can buy. If you have a 3D printer, there are models available on places like Thingiverse for adapters between most brands of facepieces and cartridges.
It also helps to use a snap on filter cover for VOC cartridges, if available, so that the cartridge doesn't become clogged with paint particles before the carbon is exhausted. I know that 3M and MSA make these snap-on style overfilters, and probably other brands do as well. They are worth it if it helps extend the life of a relatively expensive VOC cartridge.