Yes you should to ensure a tenacious bond.
As long as you sand & prime first, in most cases just latex primer applied before painting with the latex paint. If the old paint is really old, be careful about sanding, it may contain lead.
Yes, as long as the latex paint is clean and sanded to a dull finish.
Latex.
yes but you cannot put latex on top of oil based surfaces You actually are NOT advised to put oil over latex surfaces, which is why you're supposed to prime will an oil-based (alkyd) primer. Latex and Polyurethane should always have a layer of oil-base beneath it.
If I was going to repaint this, then I'd remove both the latex and the old oil base with a chemical stripper. Then re-prime it and paint.
Water based paint such as latex and acrylic do not adhere well to oil paint. In order to paint over oil paint you must sand the surface and prime it with a good quality oil or alcohol based primer and then top coat with a water based paint. Oil based paints will adhere well to water based paints sp it is unnecessary to prime before top coating, unless you have had to do any repairs to the surface.
I don't believe there is 'water based enamel' -that's a contradiction. Answer If your previous paint is glossy, trying washing it with TSP (or substitute) and lightly sanding it. Try the latex to see if it will adhere without bubbles. Give it plenty of drying time.
An oil based primer is preferable. Something like Kilz Original is excellent for this use.
You should prime before painting when you want to improve adhesion, hide stains, or create a smooth surface for better paint coverage.
For painting a brick fireplace, it is recommended to use a high-quality latex paint that is specifically designed for masonry surfaces. This type of paint is durable, heat-resistant, and will adhere well to the rough texture of the brick. Be sure to clean and prime the surface before painting for the best results.
You need to prime first, with a primer that adheres to oil paints, and to which latex can adhere. You may even want to sand the oil-painted surface lightly to scuff it for better primer adhesion. Painting latex straight on top of oil paint will result in peeling.
Using exterior paint on the interior should not be a problem. You would only use exterior paint outside because it weathers better, which is not really an issue here.