Stains on white knit after washing

There are several possible causes of stains that appear after washing—I can’t say for sure without knowing more about your situation, but here are some things for you to consider as you try to pinpoint the problem.

1. Dye Transfer. If you had a dark or colored item in the same load as your white shirt, some of the color could have transferred onto the shirt. This is more likely if the dark item is newer, or if the laundry stayed in the washer for an extended time following the spin cycle—a damp dark item squeezed against a damp white item can transfer its color more easily, especially if given enough time. White bleachable items can usually be restored with a bleach soak:  add ¼ cup Clorox® Regular Bleach₂ to 1 gallon of water and fully submerge the item for up to 5 minutes, then rinse thoroughly and allow to air dry.
2. Rust. This can happen in older washers when the enamel coating on metal machine parts has deteriorated. If rust spots develop on the machine in areas that collect moisture, then any water dripping onto the clothes can carry a little rust with it, and you’ll see brownish red stains after washing. These stains can often be removed with a little rust remover (usually available at the hardware store), just be sure to follow the package instructions, including wearing gloves. And it’s probably time to get a new washer!
3. Undiluted bleach contact. You didn’t say if you used bleach or not, but applying bleach full strength to fabric (which we never recommend) can cause yellowing that is unfortunately irreversible.
4. Worn out dryer seal. It sounded like you were asking about stains you found on an item before it had gone into the dryer, but if you noticed the stains on the shirt after being tumble dried (and they were fine coming out of the washer), it could be that the rubber seal inside the dryer is wearing out. As that happens, grayish brown stains get rubbed onto your laundry. The good news is that these stains usually come out by pretreating the stains with a little liquid laundry detergent and then rewashing. If this is the case, it’s likely time to replace the dryer.

Hopefully this information will help you figure out what’s causing the stains.