Been there and done that a number of years ago when I deided to use water base Poly-u. Here is what I did.

Get yourself some paint stripper with MEK (as eddieC states) but get the kind that is water clean up. Get some of those green scrubber pads and a bunch of rags.
I got a bundle of the white looks-like-towels from Sams. Bucket of warm water. Old paint or chipping brush. Dab on some of the remover and spread it around some
but not too thin. It will bubble the surface some. Dip a scrubber in the water and lightly rub (scrub) the area. KEEP THE SCRUBBER CLEAN. Pat (do NOT rub) with one
of the rags/towles. Repeat as necessary. Take your time and you can get the old stuff off. I did NOT harm my covering (Super Coverite). I let the entire airplane dry
for two days and sort of helped it along with some heat from a heat gun. When all is done. LIGHTLY wipe everything with denatured alcohol. Before you apply the next
finish use a tack rag.

Now, it is MY PERSONAL EXPERIENCE that water based Poly-U can be successful. I use it all the time. But, I use Varethane Diamond Floor Finish water base. I use
it to glass and fuel proof and seal Solartax (my covering of choice now). Downside might be it is not cheap. Last time I bought some (2007 or 2008) it was $42.00
a gallon at Lowe's.

Nine years ago I built a tote to carry two jugs of glow fuel from lite ply. Stained and finished with the water base Poly-U and the finish is still holding up and has most of
it's original gloss.

I have successfully used it over painted Solartex and sealed colored Solartex running both glow fuel (10 and 15%) and of course gas. My PERSONAL experience here.

Good luck with your "project".

One more thing and don't ask! DO NOT get any of the remover on windshields, windows, acetate and/or the like. Results NOT good.

Dan