If the parts were sticky, the seller/printer either is not doing his post processing/cleaning correctly or he has no idea what he's doing. A good 3d resin print should never arrive sticky. I've had several items from various 3d printers and none have ever been sticky. The sanding issue sounds more like the print is too "thin" and thus very brittle. Again this sounds like lack of printing knowledge. Super glue or an epoxy can be used. As for painting, any pai t can be used just like on any other resin project.