I've had some experience at this; here's my 'take' on master building..... Most resin casters return your master to you after they make the mold,and use one of the early castings as a master'back up' in case of mold failure,wear out,etc. Masters do deteriorate due to body work breakdown,stress during the molding process,etc whereas a resin master doesn't (if properly stored and cared for. If you are constructing a master for a specific caster,you need to communicate with him as to how EXACTLY he wants it done,as to final finish,what color primer (Yes,this can be important) smoothness,any undercuts that may need to be incorporated into the master,finish of the INSIDE of the master,reinforcement of weak spots such as window frames and hood openings,and a myriad of other things (I'm sure I've forgotten some ). Some casters buy masters,but many pay in product return (! master done,in return for copies of the finished product back,etc )Discussion of ALL of this is a VERY IMPORTANT PART of the process....(If you're casting an item for yourself,most of this doesn't matter). One other important thing you need to discuss with the caster is 'turn around' time (length of time after he receives your finished master until you get the finished product back)......Best of luck, Steve Zimmerman aka the'Z'man