Polyscale uses an agent that includes glycol ethers. I've used it with generally good results in the past but do recall that the stuff can do odd things. A gent on a railroad board had trouble with clumping when he used PS thinner (I'd bet it had alcohol in it - I've seen that happen) and here's what Testors support told him:
"Dear Nelson,
We received your question concerning the Poly S thinner. You may have
done nothing wrong, sometimes there is a chemical breakdown with the thinner and when it is mixed with paint the clumping occurs. Weapologize for any inconvenience that this may have caused you. Pleasesend the paint and the thinner back to us at 440 Blackhawk Park Ave Rockford, IL 61104 Attn: customer service. When we are in receipt of the merchandise, we will be happy to replace x2 what was ruined. In place of the Polly S airbrush thinner, we will be sending you Acryl airbrush thinner, which is an excellent product and is not known to have the same problems. <customer service persons name edited out>Testors Customer Service"
You didn't have the same problem. When I see cracking like that, I'm thinking differences in drying time. PS dries very quickly and Pledge/Future is dry to the touch in 20 minutes but isn't cured for 24 hours - real possibility that as the Future dried and shrunk it cracked the already dry Poly S. I'd like to tell you there's an easy fix here - but I'd be thinking of stripping the paint (ISP works nicely) and starting over. If you want to use Poly Scale paints it might be a good idea to track down the Acryl thinner - or at least mix the stuff in a cup before putting it into the airbrush and watch for clumping. ISP or A-20 should work, but sometimes they don't. (If I was cutting PS with ISP I'd make sure some flow retarder was in it - PS and ISP both dry really fast and tip clog would happen easily.) Unless you're using something like Vallejo Model Air and are sure the consistency is right, it's never a good idea to mix paint inside the airbrush.
I'm not sure whether using Pledge/Future as thinner would help or not. I use it as part of my brew for thinning Vallejo Model Color paints and it works nicely there. But Future is a liquid acrylic so it's closely related to the polymer agents found in paints like Vallejo - wouldn't guarantee results with solvent based paints like Tamiya but it would be an interesting idea to try if you don't mind a satin finish. Water can probably be used in any acrylic but only in small quantities - maybe 10% - use a lot of it and the paint will spray but the finish will be poor because the paint's structure will break down.
The one thing you did that I don't is to seal the acrylic base with Pledge and then put more acrylic on top. The only time I did that was when I did the P51 MKIII for this build. As noted earlier I weathered the plane completely and then put the stripes on at the end. I distinctly remember letting the plane dry at least 24 hours because I knew I was going to be masking over the Pledge and wanted it fully cured so the tape wouldn't pull anything off..
Everybody gets hit with paint strangeness sometimes. That's one good reason to stay with one brand and there's no doubt Tamiya is the most airbrush friendly paint on the market. It dries so fast that some folk have trouble with clogging or a rough finish but those problems are easily handled by thinning the stuff with Tamiya or Gunze lacquer thinner (A-20 is not as good - it works and many good modelers have learned to use it but that's testimony to skill not A-20). That will keep a nice flow and because lacquer thinner dries much slower than A-20 (mostly ISP) it helps with both tip clogging and rough surface. It also allows a thin mixture which makes Tamiya extremely easy to clean - I use 30-50% thinner in normal circumstances when using Tamiya with lacquer thinner. (Not hardware store lacquers - that stuff is much too strong and will attack plastic. Stick with Tamiya or Gunze's own brand - either work fine.) So there's good reason for Tamiya's popularity. (Gunze is basically the same as Tamiya in my experience.) I've used MM only rarely and can't comment. All other brands like Vallejo and Polly S require a learning curve. (If you don't have one get a "medical experiment" kit for use as a lab rat. I've got a Revell P-38 that's a poor kit but has helped me through many problems - you should see the color scheme it has now.) I think it's worth the effort to master Vallejo, but wouldn't hesitate to recommend Tamiya for modelers who don't want to their head against wall a few times when the airbrush betrays them.
Eric