Both linseed oil and tung oil are hardening oils; they undergo a reaction with oxygen in the air and get progressively denser until they form an impact-resistant, water-resistant film. For this reason, linseed oil is one of the main ingredients in many types of oil-based paints and varnishes. Not only can you paint over them, tung and linseed oil are both recommended undercoats for oil-based and latex paint.
Boil the Oil
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If you're using linseed oil, an important requirement is to use it only after it has been boiled. Like flaxseed oil, linseed oil comes from flax seeds, but the difference is that the seeds are cold-pressed to produce flaxseed oil and boiled to produce linseed oil. The product that comes from boiled flaxseeds is still considered raw linseed oil; it doesn't become suitable for wood finishing until it's boiled again -- sometimes twice. Boiling induces a chemical change in the oil that makes it harden more quickly. Tung oil, on the other hand, comes from the nut of the tung tree, which grows in Asia, and doesn't have to be boiled.
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Painting Over Oil Finishes
Just as you would never apply a coat of paint over an existing one until the undercoat has dried, you shouldn't paint over wet oil. Depending on the type of oil and the way it was processed, it could take as long as a week or more for it to harden. Before painting over oil:
- Feel the surface of the wood. If it feels tacky, it's too early to paint. If the surface still feels tacky after several weeks, it may have been finished with raw linseed oil. If so, rub the wood down with mineral spirits to remove as much of the oil as possible before painting over it.
- Scuff the wood with 320-grit sandpaper to smooth the wood and etch hardened oil. Once it has cured, an oil finish can be very hard, and some paints may not adhere well.
- Prime an oil finish with a high-quality latex primer before coating with latex paint. This is the same precaution you should take before painting latex over oil-based paint.