Just wanted to update the stain removal process for future forum users as to my experience.
First up was a good wash and rinse.
I used Starbrite gel hull cleaner (oxalic acid) multiple times. It was probably the most effective of all, excepting the final winner. I liked the gell product for not just running off. Not a bad product.
Then it was on to bleach and water, in increasing strength, up to straight on bleaching. Repeated this three times with no effect at all.
Tried another oxalic product (FSR) with no further brightening of the hulls. Washed and rinsed, back to the drawing board.
Bought buffer pads and hull polishing compound meant for light to medium oxidation and cleaning. Smoother surface with oxidation removed, but no cleaner.
Finally, today the winner! Muriatic acid (hydrogen chloride) available at most hardware stores for $9. I used 1 pint to 1 gallon of water for solution. Used in well ventilated outside area, had protective gloves on and wore eye protection. Just sponged on liberally and waited 20-30 minutes before rinsing well including the trailer it was on. No rubbing or brushing.
If anyone goes this route, please heed the warnings of using this product. Simple, fast and effective for a 36 yr. old cat in need of well deserved cleaning.
Before:
After: