[Improbcat]

You don't need to use POR-specific fiberglass mat, any of the chopped-strand type fiberglass mat will work.

Do all your rust grinding & surface prep, then trim a piece of the mat to cover the holes and overlap onto good metal. Lay down a coat of POR on both sides of the metal as usual, then lay the mat on a plastic covered surface and brush it with POR on both sides, forcing the POR into the weave. Lay this over the still-wet POR on the metal, and use a POR covered brush to force any air bubbles out and the mat hard against the metal. Wait for the POR to dry and put another coat of POR on both sides to seal the mat & metal together.

Be aware that you are going to have to cut out the entire POR & mat section and weld in new metal as there is no way to remove this fix from the metal for welding.