Awesome Slime Blower so far!
Maybe i could ease your mind about the spray paint.
When i constructed my Slime blower i went the same route as you did with your main tank. I did some skim coat of bondo to seal the domes onto the main tank and skim coated everything and sanded before priming. Then after the primer, i used rust-oleum grey spray paint, then finished it off with a shot of clear coat. This way no fingerprints would be visible and it would seal the gray nicely onto the tank.
Here is a photo of mine before we finished it, but its the best i could find close up and without too much light drowning out the grey color. I hope this helps
Sorry for such the large picture idk how to make em smaller