Alkyd paint has very poor alkali resistance. The alkalinity of a damp concrete surface can be quite high. Alkali attacks the resin system in an alkyd and makes it soft and sticky (saponification), resulting in poor adhesion. This situation is most likely to occur on concrete floors where there is an occasional moisture problem in the concrete. If an alkyd is saponified, it must be removed before the surface can be recoated.
Alkyd Sanding Sealers
Alkyd sanding sealers are clear coatings that are easy to sand. When solvent comes in contact with wood, it causes the grain of the wood to raise, making sanding necessary. Sanding sealers are recommended for use as a first coat on new wood or over wood stains. The materials in sanding sealers that make sanding easy can interact with polyurethane over time, forming a slick hard finish, causing peeling of the polyurethane from the sealer. Sanding sealers should only be used under alkyd varnishes.