A razor saw is the best tool for removing pieces from their casting blocks. Micro Mark sell x-acto blades with teeth that also are great to use.
I recommend to clean resin with denaturated alcohol before painting. The denaturated alcohol will remove the grease, oils, and mold release agents from the resin. Resin can be glued using CA, Testors cement for wood and metal, and epoxies. Cleaning resin should be done with a x-acto knife and wet sanding while wearing a mask. Resin dust is toxic.