paint\priming problem

I recently purchased a new driver's side door for my 87' LX. I decided to try and save some money and prime it myself. I had never prepped an already painted car, and I believe I sanded it too much. Anyway, the reputable body shop I went to told me they couldn't do the job. I haven't yet extracted all the details as to why not, but I will post that later. I will also post a picture but if anyone has any suggestions based on this information, it would be very helpful.

Tell me exactly what you did to the door to prep it.

Tell me what the paint shop said.

And a picture would be great!

I probably can help you.