What a boring post, but maybe someone can help me...
My daughter works at weekends for a company where they dress up as eg Disney princesses, superheroes, fairies, etc and run kids' birthday parties. The costumes get sent around between the girls to wear, unless you get a lot of requests to be a certain character in which case you mostly keep the costume.
She does Belle (from Beauty and the Beast) a LOT. So she has her own dress, which is beautiful. The bodice is a kind of stretchy yellow velvet with a sequin trim and the skirt is a huge swagged yellow full skirt with a lining. The fabric is a kind of satin, but very heavy and thick. Recently another girl asked to borrow it and has retuned it covered in tiny brown spots. She SAYS she washed it (from a not abnormally dirty state, just worn) the way we always do (in a laundry bag, on a delicates setting), and "it just came out like that" - which I can't believe. It looks like it was spattered with mud from a passing car and then not soaked before the stain had set. It's all over the skirt. Plainly Belle cannot be seen in such a dress (unless you notice in the new film when Emma Watson flings it from herself while riding a horse, which could explain the damage!)
Today I soaked it for two hours (double the recommended time for colours) in the exact strength recommended on the tin of Vanish. No change.
Do you have any other ideas? To recap, large surface area, yellow manmade fabric, heavy but quite delicate in terms of its sheen and likelihood to get snagged, unknown stain, and joined to delicate fabrics and sequins. Dry cleaner wouldn't touch it. Thank you for reading.