I use muriatic acid for cleaning stains, blood, rust, etc. just don't use it on aluminum , and wash it off with soap and water after the stain comes out.
Muriatic acid is a diluted form of hydrochloric acid. Hydrochloric acis is nasty stuff and can be extremely dangerous. I usually don't see a need to use it based on the potential risks. The only time I will use or recommend is cases of extensive barnacle growth in places that are hard to get to. Even in that case, proper precautions should be taken..
That said, as with most stain issues, the best approach is to start with the least caustic approach and move up as needed. In the chair case, vinegar could be a start but I would try a marine mold and mildew cleaner first. I generally don't use bleach since if left on for a while it can stain gelcoat and can damage stitching on seat cushions.
I also don't recommend things like magic eraser as a starting point. Powders like Barkeepers Friend in the non scratch version work well on many stains but like the eraser they are still abrasive. They are more abrasive than you might think and can remove protective coatings that your seats may or may not have. Using abrasives can make the seats look good for a while but if they had a protective coating and you remove it, they will get dirty again.
If the seats are in poor shape, cleaning, sanding and painting is an option. There are spray paints made specifically for plastics and the results can be quite good.
