Expanding foam under for support. Grind out shallow groove VERY shallow for fiberglass strips. Paste them in as level as possible. Level out as best as possible with Bondo. PAY a refinisher to spray it DO NOT use the home depot junk it looks better with the hole. Mask off the tub before the quote so the contractor sees he only has to paint. Its 75% of his time taken off the top. Should find a GOOD painter for $150 in that scenario. Use good masking tape, no plastic masking cuz the gun pulls it in, and buy a roll of 3 foot paper from depot. one row on floor front of tub, one row tub UP on walls or tile, cover toilet. drop cloth vanity and remmainder or floor where exposed after papering.