To paint latex over oil base is fine, but depends on many factors that can affect paint adhesion and durability. Preparation, quality of products used, speed and effort used to apply the product and temperature. Also consider stability of the surface to be painted, including shrinking, expansion and moisture content.
Obviously surface preparation is important. If the surface is dirty, oily, or too glossy, the paint won’t adhere properly. This isn’t the problem because your paint stuck to the old paint; it is the old paint that fell off the wood.
Given the fact we don’t know who or how the initial coats of paint were applied, we can’t address the quality of paint, the conditions during application, or the quality of the application. We can look at the paint chips and notice they are brittle, generally indicating oil based paint (latex chips are usually flexible). Then look for a reason why the paint pealed. Was it pushed off by moisture? This is one of the most common reasons paint will peal down to a bare surface. Check for leaks, lack of caulking, damp interiors, or any source of moisture. You have to repair the cause sometime.
Carmen Amabile at Karm’s Painting Service brought one problem caused by painting latex over old oil paint to my attention. He said latex paint could pull off the old oil paint as it dries. He directed me to the Paint Quality Institute (www.paintquality.com) to confirm this, “Old oil based primers and paint, when built up in multiple coats (say more than 4 or 5) may come loose if a latex paint is applied. This is because as it ages, the old oil paint gradually oxidizes and tends to contract, and adhesion is reduced; then the latex paint, as it shrinks as it dries, can pull at the old paint until some or all of it comes loose.”
This is not a particularly common thing, but it is just another thing that happens, and we recommend in cases where there are multiple layers of old oil based paint, that to be safe, apply an oil based primer and/or paint when it comes time to re-do. Alternatively, strip the old paint, sand the wood, and apply quality latex exterior primer and paint.
I hear stories about the best exterior latex primer and paint we could buy before it starts to peal right down to the bare wood in 6 months.
However, if the old paint is suspected of containing lead, do not remove or sand the old paint. For guidance, contact the EPA at 1-800-424-LEAD.” John Stauffer, Paint Quality Institute.