UPDATE:
I poured isopropyl alcohol (70%) into the wheel wells and let that soak for a while (hour or more). That did the trick. It's still difficult to get out, but I'm using a toothpick, Q-tips, and Micro-Brushes and that is doing the trick. It's tedious, but I'm not marring the wheel wells at all.
The alcohol softens up the latex, and breaks it up to the point that it comes off the plastic quite easily. Far, far better than peeling off the mask "dry".
That liquid masking agent will NOT, I repeat WILL NOT, be used for wheel wells any more! It will stay on canopies, and that's about it.
On Bench: AM P-51B, Tamiya 1/48 F4U-1A
On Deck: Hasegawa F-14's (too many); Tamiya P-47D; Academy P-47N;