Test an inside seam with bleach for colorfastness, making sure it doesn't bleed through to the outside in case it's not colorfast. Blot it with a paper towel and check for color. Let it set and watch it for a few minutes to see if the color starts to fade. If it doesn't, try a little bleach on the white part of the shirt to see if the stain comes out. Read the whole reply before trying the bleach. Since it's been outside for a long time it's hard telling what the stain is. Start with the least aggressive approach first and work your way up. You've tried low aggression already. Now try different cleaners with a toothbrush and hot water on small spots until you find one that works. Things to try - dishwashing liquid, lye soap, white vinegar and baking soda, lemon juice, ammonia, rubbing alcohol, acetone, solvent type dry cleaning fluid such as Imperial Cleaner (may or may not be available in your area, it's made in Kansas City), kerosene is close to the same thing. Try each one individually and rinse before adding another. Use in ventilated area.
Remember, chlorine bleach damages wool, silk, leather, mohair and other protein-based fibers. It also harms nylon and spandex. It will yellow and weaken them. Acetone harms acetate, triacetate and modacrylic. Cellulose fabrics are more vulnerable to acids, and protein fibers are vulnerable to alkalies. Polypropylene (olefin) is damaged by perchloroethylene, the dry cleaning solvent used by or previously used by dry cleaners.
Credit must be given to Cheryl Mendelson for much of the above paragraph which is taken from her book "Laundry - The Home Comforts Book of Caring for Clothes and Linens."
Soak the stain in cold water for twenty minutes. Work liquid laundry detergent into the area and allow to stand for thirty minutes. Rub with detergent . Rinse. If you suspect that it might be rust, treat with rust remover before using bleach. Bleach will make rust stains worse. Launder in the washer using regular cycle with hottest water the garment will stand. Silk and wool cannot take chlorine bleach and should be soaked in warm water and agitated very briefly, if at all. Air dry and block if shrinkage is a concern.
Soak the stain overnight in an enzyme presoak. Launder.
Sponge the stain with drycleaning fluid. Let stand for twenty minutes. Rub with detergent. Rinse thoroughly.
If the fabric can be bleached, mix equal parts liquid chlorine bleach and water and apply with an eyedropper. Do not use on wool, silk, spandex, or noncolorfast items. For these fabrics, sprinkle oxygen bleach on the stain and dip in very hot or boiling water. Launder immediately.
If the stain remains after all these steps, nothing can be done to remove it.
The above was reprinted with minor alteration from "Stain Removal from Washable Fabrics," by Sharon Stevens, Dept. of Textile and Apparel Management, University of Missouri-Columbia (1993).
On a personal note, I would continue to wash the shirt repeatedly in hot water with detergent and STTP or a phosphated detergent if available and the stain my fade over time. The color might not be as bright, but eventually it may come clean. You could also try mixing up a strong solution of phosphate and detergent in a stock pot in hot water from the tap, put the garment on the stove and bring to a slow simmer, check and stir it occasionally to see if the stain is shifting. If it comes out, move the pot to the sink and slowly dilute the boil wash with hot tap water. Pour out most of the water, take the pot to the washing machine, pour the water and garment into the machine and spin out.
Carefully disregarding care labels is an art learned over time. I read them once and then decide how to proceed. If I forget I will refer back to the label and then do as I see fit, depending on the price and how much I value the garment.
