Oil Paint vs Latex Paint

How to Identify Oil Vs latex

Is It Latex Or Oil On The Walls?
When there is an existing painted surface that you want to paint over, how do you determine whether the previous paint is oil or latex? We've had some problems in our house with earlier work painting latex over oil with no primer. We'd like to use latex but we don't want to prime unnecessarily (don't want TOO many coats of paint on a surface). Is there a quick and easy technique to identify oil vs. latex?

Water-based (also called "latex") paints These offer the easiest application and soap-and-water cleanup, are the fastest drying and have less odor than solvent paints. Latex performs well on interior walls and ceilings; its quick drying time prevents the kinds of ripples and sags that can occur with slower-drying alkyd finishes. For brick, concrete and cinder block, only latex should be used.
Alkyd (commonly called "oil-based") paints This type generally cost more than latex. (Though interchangeably referred to as "oil-based", they're not the same thing as the traditional oil-based paints that they have largely come to replace.) Application requires patience--thin coats are generally necessary to avoid problems like sagging and streaking, and the drying time is usually longer than latex.
The up side is a paint job that stands up beautifully to long-term wear and tear. Scrubbing off dirt and scuff marks won't harm the paint. Alkyd paints are the superior choice for painting floors and metal surfaces.
Tip: Remember, oil and water don't mix. For best results, you should consider applying the same type of paint that's already there. (Not sure what's there? See below) Consider the following rules when choosing the type of paint you'll use.

For interiors: You can safely apply latex over oil, or oil over latex, but only if the previous finish is flat or if it has been sanded. If in doubt about what's there, you may want to cover first with a primer of the same composition (alkyd or latex) as the paint you choose to use.
For exteriors: Oil should not be applied over latex. The best exterior combination is latex paint over alkyd primer.
Is It Latex or Oil-Based Paint If you're ready to do some painting over old paint and wondering whether you need a latex or oil-based paint to cover it here's how you can easily find out.
* First scrub a small area with a solution of houshold detergent and warm water. * Rinse well and towel-dry it, then soak a cotton ball, Q-Tip or soft rag in alcohol and rub it back and forth over the cleaned area. * If paint comes off, it's latex paint and another coat of the same is in order. * If the paint doesn't come off, it's oil-based, and a pre-coat of oil-based primer is a must. * Then you have the option for the finish coat since different types of latex or oil-based paint can be used.