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How oil-based paint works:

Certain types of oils (e.g. linseed oil) harden when exposed to air. Add some pigment and you have paint. Easy. Oil works as both binder and solvent.

How water-based paint works:

Water is the solvent. It makes the paint soupy, but it doesn't leave anything behind when it dries. Something else is needed to form the hard skin. In water based paints, the hard skin is made of plastic. Plastic doesn't dissolve well in water, so it is encapsulated in little polymer balls that are water-friendly on the outside and plastic-friendly on the inside. "Latex" is a general term describing a bunch of little polymer balls floating around in water.

How Shellac works:

Shellac is an insect excretion scraped from the bark of trees in India and Thailand and it has been used for 3,000 years.

How opacity works:

A clear film is useful for some things, but to give paint some substance, they stir in titanium dioxide to make it white. Remember when people used to put that on their nose to protect against sunburn? Same thing here. You want to keep the light from going through. They used to use lead, but that got shut down for environmental reasons. The more TiO2 you have and the more finely ground it is, the better the paint will cover.
 
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