The last board I glassed turned up with a thumb print sized dark mark after lam. I was using white pigment and it still showed through. From now on I watch the supplier roll the glass out if I’m buying less than a whole roll and make sure they wear gloves.
besides what the guys have already told you regarding glass from factories and new gloves(rinse them before using to get off that dust they come with, sometimes that will stain but thats more of a yellowish color, at least the ones i use.)
are you using any sort of cleaning solution like acetone or something?? if you are and it’s dirty, drops of it on your squeege will stain the glass/board during the lamination process… or drops of it can drip off your glove and stain it too. also check your bucket too, make sure the inside is super clean. stir stick too! everything has to be checked to make sure you have nothing that can transfer or “melt” into the resin/epoxy bucket and stain your lamintion. also check the table where you are mixing, maybe your picking up some crap on the bottom of the bucket and it’s dripping onto the board.
these type of things can drive you insane till you figure out what the cause is.
good luck and tell us if you ever figure out what it was, i would love to know!
I have had some smokey looking stains like swirls running across some s glass I have used lately. It was definitely in the glass as I sanded some of it out and reglassed. It was on sections of the cloth that were not the outside of the roll. Doesn’t show up often and is only about 6-8" wide running rail to rail. Knock on wood but I haven’t seen this on the E cloth I use from the same manufacturer.
When I’m not glassing I always cover my glass rolls in the rack with a beach towel to keep any random dust from landing on them.
No matter where or how clean you think you’re being, a layer of dust or bits of whatever will settle on the roll and, of course, only show up after the resin hits it.
Old latex gloves rot or perish and turn pink/brown, that can transfere to glass, happened to me a few time if I picked up an old pair of gloves, try switching to nitrate gloves.