Nah a marble or rock won't block the tube, the tube sits near millimeters of the bottom (Well it should in most jars)...So it wnt get in the way unless you use one thats that tiny it wouldnt do anything anyway..And yer paint clumps after awhile of storage, nature of gravity and other chemical and molecular attractions but thats why you put a rock in it..If its good enough for a spray can its good enough for an airbrush as a spraycan has just as fine if not a finer nozzle, as the can says, agitate well..Do the same with your stored paint and it does break it up extremely well but shake it well..White can be a pain as that really likes to clump when stored..You can purchase tiny strainers that go onto the actual tube of the siphon feed, not sure if thats what ya linked but I've done this for years with various paints..Clumping in paint isnt an issue if you de-clump it again prior to use, the worst thing about storing paints, especially for extended periods and if you have reduced them with water is actually bacterial growth, that can make you sick so if the paint is stored for ages and has a weird smell when sprayed, don't use it..But thats why you put them in the fridge or a dark cool spot if you can as it deters that bacterial growth..
Its simple really, if it can be stored in its actual container you purchase it in...It can be stored in the airbrush container....
Just checked the link and yer thats what I was talking about but that one does seem quite course and may still let smaller clumps through (Bit hard to tell in a closeup pic), but it would certainly help
