Oil vs. Latex – Intercoat Adhesion Problems
The average homeowner cannot tell if the existing coating on the trim of their homes, as well as the walls and ceilings in their kitchens/bathrooms is an oil or latex coating just by looking or touching it. However, it is important to know before applying a new latex coat of paint because applying latex over old oil finishes is guaranteed to result in intercoat adhesion problems – the new paint will scratch right off the old finishes.
Intercoat adhesion problems arise when a latex paint is applied over an older oil paint. Paint needs something to grab on to when it is applied, and the old oil paint finishes (especially semi-gloss and full gloss finishes) are not very porous. The result is that when new latex paints are applied to old oil finishes, they do not form the proper bond to resist scratches and nicks.
The solution is to create a more porous finish on the existing oil paint, then apply the new latex paint. Doing so provides a good “bed” for the new paint to bite into, thereby eliminating the intercoat adhesion problem.
There are three common methods for creating this more porous finish prior to the application of the new latex paint. First, there is scuff sanding. This involves using fine grit sandpaper and running it over every square inch of semi- or full-gloss oil paint to roughen up the surface. The upside of this method is that it does not require an intermediate coat of material/paint/primer, and so reduces the materials cost for the project. The down side is that it is easy to miss minor areas.
The second method is to use a deglosser. Special deglossing chemicals have been developed to be wiped on the semi- and full-gloss surfaces. These chemicals open up the pores on the surface of the old oil finish, providing a nice bed for the new latex to bite into. As with scuff sanding, this process does not require a full coat of material to be applied, thus saving you material and labor costs, but it uses noxious chemicals and minor areas might be missed.
The third method is to apply a specialty primer to all the semi- and full-gloss areas. These primers are formulated to bite into the brittle oil finish, as well as provide a porous bed for the new latex. This will give you the best performance over the longer term, but it involves having a full coat of a specialty coating applied, and most likely applied by hand. This means more costs for both materials and labor, but will ensure that the new finish will not scratch off easily and will be durable for years to come.