Polyurethane resin casting is flaking

Hi all,

I'm trying to slush-cast a few parts of a Ghostbusters proton pack in polyurethane using RTV silicone moulds, but I'm having trouble with the built-up layers of PU resin not adhering to one another.





Below is the setup I'm using for another part of the proton pack. I think the silicone was flexing a little as the PU cured, which meant subsequent layers were moving relative to one another, so I've since increased the thickness of the silicone and made a new mother mould out of fibreglass.

I've yet to try casting this in PU resin - I was going to try gelcoat followed by polyester resin and fibreglass, but I'd prefer to use PU if possible.



Below is a pour of PU in the original mould.



I'm using Fastcast polyurethane resin with black pigment added to it. I tried casting the item in the first picture without adding any pigment and it still came out a bit flaky.

Any suggestions are gratefully received!