Scrape the seam with a single edge razor, normal to the seam line. Much faster than sanding. Finish with a 320 paper on a block. You will go through a lot of blades but this is fast. Brush prime with PPG K36 if you can get it

If it is still just the glass (no formers installed) I would take the thing into the shower and wash it inside and out first. Look out for sharp edges. In any case you got to clean it with something. Now is the time