> Ok then, it is probably a characteristic of dry film negative resist. Of
> the 3 different brands of negative dry film I have used, all form a
> lightly adhering 'mushy' coating that needs more than just light
> agitation to be removed. Once the PCB goes in the acid etch, any
> photoresist residues immediately become solid again, leading to shorts
> between traces. Its really frustrating when that happens.
>
That "defect" of negative dry film photoresists is really an advantage to be reliable and stronge. If you clean, laminate, and image properly, the dry film adheres so well that it can be handled with security and without peel-off risks.
The kind of fabric that you point in your first link would be useless since it is too soft. You should use a real sponge with medium abbrasive action.
One solution is to make an spray developing tank, which is easier and not so critical than a spray etching tank.