That's the trouble with models like that... the resin is very rare to find as spare parts and if you buy a used one, there's no telling until it's in your hands how resilient the resin actually is. I bought a DW-5700 that had the resin intact. It looked a little worn, but it wasn't bad. After a battery change, I noticed a slight hairline crack in the bezel. The pressure I'd exerted on the resin to get the back off must have caused it. And then when I washed the watch, that was all it took--the crack grew and the bezel fell apart.
Nothing you can do. No treatment to the resin can reverse the rot, unless there's some secret method that hasn't been leaked to the general public. The only thing you CAN do is to protect existing resin to help delay the onset of rot. I can appreciate the frustration you must feel for those watches... and the 5700 is not much different, because CASIO in its strange inclinations made subtle changes to the 5700, resulting in 3 different bezel designs. Two of them are physically different and the third has different function label positions. So, despite having secured a vintage 5700 bezel, it doesn't work on mine! I had to put a 5600 bezel on it... which works, but significantly alters the look of the 5700 design.