Thanks so much. I was pretty happy with the appearance of the japan, myself. In fact, my first two batches came out beautifully, but they were mostly quarter plates. I really started to notice the issue on the next batch, which included half plates and 5x7s, but in my last batch, it happening even on the quarters.
I was using #00 steel wool to roughen up the plates, followed by a wipe-down with mineral spirits.
I'm going to give your solutions a try. I'll pick up a green Scotch Brite pad in town today, if we don't have one under the kitchen sink already. And I'll lengthen my pre-bake drying time. (I'm flowing the stuff on.) Here in West Virginia, our humidity is pretty high. I'll double my drying time from 2 hours to four hours and see if that helps, on this next batch.
For the ones where the sags aren't as pronounced, I thought I might give them another top coating and see if they can be salvaged.
It might sound silly, but at this early stage in making the plate, watching the second coat do this to the first almost as heartbreaking as having your varnish eat your collodion.
Nothing like a little pressure to get this right... I have an event this Saturday.