Can I sand to remove old finish and stain to prep a fiberglass door, If not then WHY!
Never sand an embossed fiberglass door. The embossing is the wood grain effect that these doors have. Sanding will remove the embossing, damage or even remove the top layer of resin, gel coat, and expose the actual fiberglass.
A fiberglass door can be ruined by aggressively sanding to remove the finish and stain. Use a fiberglass safe paint and varnish stripper instead.