Questions for the painters...

I'm ready to epoxy the underside, interior, trunk, and engine bay with DPLF.

I would like to spray epoxy, then seam seal, then topcoat. There is some pitting where I blasted rust, and there are some plug welds that aren't perfect. Would a couple coats of K36 or Slick Sand before the seam sealer be acceptable? I have never used it on the underside. Would it be durable?

Also, where on the front frame rails does the body color end and the grey primer begin?

Thanks!