I would be very willing to bet that it's emulsion. We have this same problem at my day job now and then (specifically in the warmer, more humid summer months). We only print with plastisol. I figured out that at our shop the humidity is pretty high in the summer which was leading to the emulsion not being bone dry before we expose screens. This led to screens being slightly underexposed in the summer. Basically the inner bit of the layer of emulsion isnt quite dry enough and doesn't expose properly. So if we go right to press with those screens after they dry out as good as they could in our humid shop the ink was reacting with the underexposed emulsion and causing this super thin hair line of emulsion around the image area that wouldn't come out during reclaim. We also don't use a pressure washer, but we don't let the emulsion remover sit....we scrub it around like Luther was saying so it doesnt dry. Anyway, the reason I concluded it had to do with underexposed emulsion that wasn't dry enough was because I started leaving the screens outside in the sun to dry after exposure and pick up a post exposure from the sun while it was drying...or if it was raining or super humid outside I started leaning the screens up next to where our flash dryer is to dry out in the heat and then threw them back on the exposure unit after for a post-exposure. Both of these methods got the emulsion bone dry all the way through and fully exposed. The problem disappeared. In the winter when it's super dry in our shop we don't have the problem at all either. Humidity was causing the problem. I bet this is your problem to. Try drying your screens out in the sun or near some heat after exposing them and post expose 'em.
What you just described Gein sounds very very similar to my problem, perfectly clean screen aside from a hairs-width outline around where the stencil was. Did you have any luck getting the screens that were damaged clean? Or did you simply re-stretch them?
I just invested in a power washer, so i'm going to try and black the hell out of it later.
Some of the coarser mesh screens that we use (110) I was able to take a needle from our embroidery machine and sort of poke the blockage out....it took forever, but I was bored that day. Haha. The higher mesh counts like 196 and 230 were pretty much lost causes...I just used them for designs that I could place on the screen in areas that weren't affected. I'm pretty sure that if we had a pressure washer it would take care of it. But since we started making sure the screens were bone dry and post exposed before inking them up we haven't had the problem anymore.
Pressure washer blew out parts of the damaged screens, but not enough, my water bill would override the value of just getting the rescreened if i'd of carried on. After my print run of 150 today, i pressure washed, post exposed my screen like Gein said and everything came clean perfectly, so i'm assuming your hunch is correct Gein! Thankyou!
I've been dealing with this headache for a long time, but we've switched emulsion/remover/inks so many times it's hard to tell where the problem lies.
We have a pressure washer and our emulsion remover is CRAZY (we can blow out emulsion using the pressure from the spray bottle), and we have used Permasets from time to time, so maybe it's an ink thing.
I do find that the tiny 500mL bottles of Permaset ink are really shitty, and they get really gakky really quick.
EDIT: +1 for post-hardening after exposure/washout. We generally let screens sit overnight after washout, and then harden them the next morning.