Yes, you can.
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.
Yes, as long as the polyurethane is oil-based.
Yes, both oil and water-based polyurethanes will adhere well to an item coated with acrylic paint. If it is a polyurethane paint, dull the surface prior to painting. If it is a clear. realize that that oil based polyurethane will impart an amber hue while a water based polyurethane is crystal clear and will not affect the color.
If you mean an oil based polyurethane, no, it will dry too hard and will chip off over time and normally isn't meant for exterior work. A better option would be to use a 100% acrylic clear polyurethane, paint or solid stain. They are adhesive enough to adhere to vinyl fencing or decking and will flex during freeze/thaw cycles.
PU stands for Polyurethane, a kind of plastic that's the main ingredient in the paint.
Because it will impart a yellow cast to the paint surface.
Yes. Clean and degloss the oil based paint, apply an oil based primer, then apply the water based paint.
That's a common procedure, I haven't had any problems doing it.
Yes, you can paint a floor that was stained. However, before you apply any paint you will need to apply a coat of primer/sealer so the new paint has something to adhere to. Also, by sealing the floor you will increase exponentially the life of the paint you want to apply. Otherwise it would wear away very rapidly.
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. Clean and degloss the oil based paint, apply an oil based primer, then apply the water based paint.
Apply some non-slip floor paint