I believe your problem is the proportion of water to ferric nitrate
you are using. I use 1 lb of crystals to 2 cups of distilled water.
You can use less of the ferric nitrate and get a slower etch. I use
the stronger solution and watch my etch closely when the solution is
fresh (check after 30 min as it may be done). It seems to me that the
resist–PNP Blue–holds up better with a strong quick etch than with
a slow weak etch. Of course as the solution is used, it continually
degrades and you need a longer and longer etch time, until you need
to make a fresh batch of etchant.
Good luck with your project!