Painting water based latex paint directly on top of oil based paint without proper preparation will result in a bad paint job that will not adhere properly.
Water based latex paint will bend and flex a bit as temperatures and moisture levels in your home change.
Oil based paints are brittle and will crack, causing anything on the surface of the oil based paint to not adhere properly and usually chip off or also crack.
Many people prefer water based paint instead of oil based paint because water based paint is low VOC (volatile organic compounds = gasses that are bad for your health), less smelly, more earth friendly, and easier to clean up.
For people wanting to use a water based latex paint instead of stinky oil based paint to repaint doors, molding, cabinets, window sills, or other interior surfaces, follow the steps below to ensure that your paint will stick and not crack.
photo courtesy of Photocapy flickr.com/photos/photocapy/146214216/Test Your Existing Paint
Before painting, test the existing paint on your item to determine whether it is oil based or water based paint.
Soak a rag in denatured alcohol and rub in on the surface of your previously painted item.
If the paint on your items starts to come off or soften, you have a water based paint job and you can paint as you usually would.
If the paint is unaffected by the alcohol, then your paint job is oil based and you need to take the proper precautions listed below.
Is it Cured?
You must have completely dried and cured oil based paint in order to paint over it with water based latex effectively.
If it is not cured, then you must use oil based paint.
Curing takes 1 week to 1 month to happen fully. To test, you can scrub the paint with a lot of force - if it is not affected by the scrubbing then it is cured.
Cleaning
Proper cleaning is essential to get rid of the dirt and oil.
Don't skip this step before sanding else you will be pushing the oil back into your item.
You can use plain soap and water or specialty cleaners to clean your existing paint job.
Fix Any Holes or Cracks
Fill in any holes or cracks in your item using wood putty #ad.
Make sure the filler is completely dry before sanding.
Often filler will shrink when dry, so you may need an additional coat.
Sanding
Use a medium grit sandpaper (80-100 grit) to sand your existing paint job and remove the gloss from the surface and make your item smooth.
You do not need to remove all of the paint, but you do need to create a surface that is not shiny.
If you have a home built in the 1970's or older, make sure you test for lead in your paint.
If you have lead based paint, seek the help of a professional before disturbing the paint in any way.
Clean Again
Make sure to clean again to remove all sanding dust.
Any dust not removed will end up in your paint, so make sure to do a good job.
Use TSP #ad to help clean the surface very well.
Prime & Sand Again
Use a good quality primer to cover the entire surface you are wanting to paint.
The primer will help the new paint bond to your existing oil based paint.
Use a fine grit sandpaper (120-220 grit) to sand your primer to make your surface smooth, if you desire a smooth surface.
Paint As Usual
Paint your item as usual using water based latex paint.
Once the paint is dried and cured, it should adhere to the existing paint job without problems.
