If you are having a classic painted, that work is "fill" work.
As far as promised completion time frame? There is not ONE body shop that I know of that can promise AND deliver a car as promised.
People get caught up with these reality programs where a POS comes in and within a week, it's factory fresh. NOT saying you specifically had this thought of intention in your head when you took your car there, just that these days, thats what most people believe and think.
I personally know of a car that has been sitting at a shop for 2 plus years. whether it is a motivation issue with the body shop, a money issue with the car owner or a combination of both...I don't really know. What I have been told from a friend who works there is "dude, you really don't want Todd painting your car"...Then I thought....and realized. Todd is one of the best in the tri county area and is in demand...and a perfectionist. Perfection and quality take time. If I was shelling out 6-10k on bodywork and paint material...take as long as you want.
Bottom line, spring is the busy season for ANY machinist building an engine (know from personal work experience) and same with body and paint work. Many, if not all machinists/painters will take on as much work as they can plus 10 cars just so they know they have money flowing in. What is not taken into account is the insurance jobs coming in...where the REAL money is made and needs to be handled FIRST and foremost and the guy with a "toy" is pushed to the back of the line...
Just how it is man.
Solution number one..deal with it. Solution number two, deal with it...solution number three...go get your car as-is...and deal with trying to find someone to finish what was started....and wait another 3-6 months.
Essentially, the shop didn't give you a "deadline". They gave you an estimated time of completion. Body shops don't lie...they bend the truth while filling in your job when they can. Essentially, if you push them hard...and you aren't satisfied, they can retort with...."well, you wanted it done by such and such a date"...and you got it.
Then where will you be? What recourse will you have? In my professional opinion, very little to none.
My advice? Suck it up and let them finish it on their terms and time. if something isn't right, you can have it rectified satisfactorily without the shop stating..."you pushed us".
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