- when repainting exterior surfaces with heavy "chalking" (chalk is the powdery substance that come off on your hand when you run it across the surface); and
- when repainting any exterior or interior surface has four or more layers of old oil-based paint (the number of layers can often be determined by removing some paint chips and examining them).
There are also circumstances in which you should never apply oil-based or alkyd paints. For example they should not be applied directly to fresh masonry, nor to galvanized iron. In either case, the result will probably be a very quick failure of the paint.
If you decide to use oil-based coatings, be aware that they are more difficult to apply and clean up after than latex paints. They also take longer to dry-sometimes, 24 hours or more- so you cannot apply a second coat as quickly as you can with latex paint.
Oil-based paints can be used for certain applications within the home- for example, on interior trim. But keep in mind that these paints have more noticeably more odor than latex paints. That, combined with the slow dry time, may put your rooms out of service of a short while, If you use oil-based paints, you will also have to use paint thinner to clean up drips and equipment, which means that you must use extra care in handling and disposing of rags.
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