Painting Old Kitchen Cabinets - Part 3

Adhesion
A coating of paint bonds to the substrate by either gripping the surface (mechanical ), or being attracted to the surface at a molecular level (chemical). A chemical bond is made by a coating that has molecules that are extremely attracted to other molecules. This attraction depends on how ‘energetic’ the molecules on the surface of the substrate are.

This can be demonstrated by seeing how much a coating either spreads or ‘wets’ a surface, or beads up like water on a waxed tabletop. Inert substances like glass are hard to paint onto because the molecules present a low-energy surface, whereas a clean metal surface is more chemically reactive and therefore more attractive to paint. Mechanical bonds are easier to understand. For instance, a wood primer works by soaking into the tiny pores and gaps between the fibres of the wood, and then setting hard. It is difficult then to pull the paint away from the surface. Mechanical adhesion is affected by anything that gets in the way of the bond like grease, dirt, etc. Thats why cleanliness of the surface to be painted is so important.
......so, back to our kitchen doors. A hand-painted kitchen is just like any other painted surface in your home. As long as it is clean, dry, and has any glossy surface lightly rubbed down, a new coat of paint will adhere satisfactorily. Factory type finishes, either paint or a foil, are hard and unreactive finishes. The chemical resistantance and toughness of factory paints are just the sort of thing to stand up to a kitchen environment - exactly what makes them so good for kitchen furniture! Unfortunately, what makes the paint so durable is exactly what makes it difficult for another coating to stick to it. On the one hand the surface is smooth and does not provide anything for a coating to grip it in a mechanical way, and on the other, the very chemical resistance or inertness of the paint means that other paints, even those of the same type, are not attracted to it at a molecular level. If you want to paint these doors, as well as cleaning the surface, you're going to have to do some serious rubbing-down with sand paper to create some mechanical adhesion. In fact, it is a good policy to always sand any surface to be painted for this reason in a belt and braces sort of way. But even after sanding the low chemical adhesion may mean a new coat of paint may not stick too well and might "chip" off at the edges when the door gets knocked during use. One answer to this is to go back to step one and strip the existing finish off. But here's another problem. The chemical resistance of the factory paint means that its also resistant to paint stripping chemicals! Not completely, but its definately very hard work
Adhesion Promoting Primers Alternatively, there are now "adhesion-promoting primers" on the market which are designed to stick to “difficult to paint” surfaces. These primers are designed to be attracted chemically to smooth low-energy substrates. In practice however, and depending on the substrate, the adhesion of these primers can be pretty variable. Sometimes a good scratch with a finger-nail can be all that is required to scrape them off. All sorts of claims are made by manufacturers that their primers will stick to anything, and in general you get what you pay for, starting with expensive epoxy based primers. The only advice I can give before you launch into priming the whole kitchen is: test thoroughly! One good test consists of painting a small amount of the primer onto the substrate and letting it dry and cure according to the manufacturers recomendations (which could mean 14 days). Then with a sharp knife score a series of cross-hatch marks in the primer and take a piece of masking tape and press it firmly onto the primer. Pull the masking tape off again and see how much primer comes off with it. If more than, say, 10-15% of the primer comes off with the masking tape then its not really sticking well enough.