That's a common procedure, I haven't had any problems doing it.
You can apply "oil" based polyurethane over "oil" based enamel, you can also apply "water" based polyurethane over "water" based enamel, never mix the two, there's to much of a chance for problems.
It isn't so much the type of paint, its the finish on the paint, you want a tough durable finish for a Bangle. TIP: if you use a oil based enamel, use a oil based polyurethane finish, if you use a water based enamel, use a water based polyurethane, Make it a rule of thumb and you will never have bubbling or peeling problems.
Yes, you can.
I use oil base enamel with oil base polyurethane. Tip: Make sure you allow it to dry completely before you apply top coat. Good luck.
If you are painting a residence the chances are your enamel is a water borne acrylic enamel. Conversely, if you are referring to acrylic and enamel spray cans then the answer is yes you can spray one to another without any problems. Always be sure your paint is completely dry before applying successive coats.
Yes, that was the original intention of urethane.
you can paint over latex paint with latex enamel.
Latex paint is an indoor paint, which will not work very well on siding; you can paint it on, but it is going to peel off pretty quickly. Use an enamel paint.
What is Acrylic Urethane? What makes it different from Acrylic enamel and Urethane or Polyurethane paint? How does it compare in terms of application and especially hardness to the other two types?
No, always apply a primer coat to bare wood.
Yes, enamel paint will go over flat paint.
Yes, you can,and it's very good for that use. However if the concrete is bare, you must apply an epoxy primer before the polyurethane finish.