A couple things I have learned when using the Stewart system.
If you think it might be silicon plastic don't used it. I used a large shringe to measure a small batch of top coat, the plunger was silicon rubber, it fish eyed like crazy.
spray the top coat in very light mist coats, and allow 10-15 minutes between coats. it will not shine when first finished but will flow as it cures and gets shiney wet look as it hardens.
Don't use tap water to thin, it is ok to use when you are cleaning up, but distilled water is better as a thiner.
The primer for metal, is very easy to use as it comes from the can. but my gun lked it a wee bit thiner so I added a tiny bit of water, and it worked great.