That's interesting about the bubbling issue with the brightsides. I used rustoleum on the topside in a few places on my boat and it did the same thing through this long rain we just got. I'm not sure it has to do with the primer under or not? About half of it was primed with Totalboat primer and I don't think it blistered as bad. Where I noticed it the most it was over Zissner primer from Lowe's. It went away once the weather dried out a bit. I was thinking I'd have to go with better paint because of it but if interlux is doing the same thing....
In the boot stripe however, I also went with Rustoleum. I did black. It cost $4 for a pint at Walmart. I cleaned the gel coat with acetone and brushed it straight on with a good brush very wet coats. It flattened out completely with a sprayed look, is very shiny, durable, and I've seen no blisters or any problems whatsoever. So my advice? Don't overthink it. I spent less than 5 bucks and did my boot stripe and shear stripe in an evening and a professional couldn't tell the difference.