Body filler in the trunk?

Yes, grind and use a die grinder with a carbide bit to get into the nooks and crannies of the weld or where the larger grinder will bridge, leaving un-ground areas. I like using a cylinder shaped carbide bit as the edge is sharper than say a cone shaped bit.

Don't over do it. You will never get it perfect. It never was perfect. Just try to make it smooth.

Don't dismiss the suggestion of seam sealer either. Use tape to make a professional looking seam and match it from side to side. Fusor 800 is an excellent direct to metal product, though I suggest a good epoxy primer first then seam
seal right before paint. It can be painted while wet.