I recently printed some T's and when I opened the heat press, there were brown spots that were not there when I printed the shirt. It can't be ink from the printer or I would have seen it. Some spots are 1/8" around or so and one is over 1" across.
Scorch out helps some, normal ink removal chemicals for screen printing don't touch it.
Does anyone know what it is, or how to mitigate this? Now the shirts are not returnable since they are decorated.
It is likely excess sizing. Try spraying with a light spray sizing. It is like a starch but milder. It will help in some cases. I can guess the brand of tees as we've had a lot of problems with one brand in particular. The spots do go away in the wash but it kills the customer presentation. It has gotten so bad with one brand in particular that we've virtually stopped offering their shirts in white.
Spots on yellow or all colors? I sometimes have that kinda issue when surface of heat press bulds up excess ink. I always use anvil 976 with great results : )
Spots on yellow or all colors? I sometimes have that kinda issue when surface of heat press bulds up excess ink. I always use anvil 976 with great results : )
These were Anvil 976's! I've had it on Gildan's too.
I had this problem myself, humidity in an acidic or to basic environment could cause shirts to be stained ( bad bleaching of whites also might be a factor), after heating oxidation becomes more apparent in the form of spots or yellowing of the seams and sometimes the whole shirt, most of the times is due to bad storage, in origin or in house, if it happens straight from the sealed bag you should ( if can) ask for a refund. Usually whole batches suffer from this if it's a supplier problem. I have resorted to drying by heat gun if it's not to much of a time waster, since pressure is an added factor to heat in the occurrence of this stains. hope it helps
Ive noticed this too on some of the gildans Ive had. You dont see them until you press them.
To me they look like something was splattered very finely all over them...and Ive had whole orders like this. They are kind of a light brown flecking? They dont wash out either...Ive tried that.
Every time its happened, Ive called my supplier and told them about it. Seems they have had the problem before and took them back every time or replaced them.
Spots appearing after heat pressing shirts 
