Here is an article on the subject. Click here for the "Basics of Basics" Large flat panel repair.
Tech69 recently posted a thread with some tips as well. Click here for Henry's thread.
If you read the article basically what the biggest problem is on something like this is the panel flexes when you sand it. You may be blocking it "flat" but the panel is flexing flat while you are applying pressure, when you aren't pushing it flat, it isn't flat anymore!
Read the article and watch Henry's video and see if something doesn't spark.