The rule of thumb for painting is that if the surface you are painting is oil-based then use an oil-based paint to paint over it and if it is latex paint then use a latex paint to paint over it. Applying latex paint over an oil based paint and vice versa is not impossible but you must follow these 3 steps before proceeding.
Step 1- Determine if the original paint is oil-based or latex
There are a couple ways you can determine the original paint on a surface. One way is to get some denatured alcohol and a few cotton swabs. Dip the cotton swabs in the denatured alcohol and rub the swabs on a small section of the surface you are painting. If paint starts to come off onto the swabs then the surface has latex paint. If no paint comes off then the surface has oil-based paint. Another way to test the surface is to grab some medium grit sandpaper. Run the sandpaper on the surface, if dust starts to fall off then the surface has oil-based paint and if the sandpaper starts to pull paint off of the surface and the sandpaper itself has paint on it then the surface is a latex paint. Now that you’ve determined the type of paint on the surface it is time to move to step 2
Step 2- Prepare the surface
Now that you have determined the original paint on the surface you plan on painting it is now time to prepare the surface. Quality preparation work is key in any painting job. The prep work for applying the latex paint over the oil-based paint is quite simple. First you must sand the surface. Start by using 150 grit sand paper followed by 220 grit sandpaper to get the surface nice and smooth. Next remove any dust that may be on the surface and clean the surface with warm water and a rag. To prepare the surface for applying oil- based paint over latex paint simply clean remove any debris that may be on the surface and wipe down the surface with a cleaner.
Step 3- Prime and paint
Priming the surface is by far the most important step in the process. Primer will give the latex paint or the oil-based paint a surface to adhere to. When applying the oil based paint over the latex paint make sure you prime the surface with a high adhesion primer. When applying the latex over the oil-based paint make sure you use an acrylic-based bonding primer. Now that the surface is prepped and primed it is now time to apply either your oil-based paint or your latex paint.