Re: The chemistry of paint, i.e what makes it stick to walls etc.

Dear Jen,

Clearly I can feel with you the frustration experienced on searching for 
the secrets behind an everyday article like paint. One reason is the 
enormous variety in COATING formulations that exists today. Trade secrets 
also are important in this area, which developed largely by empirical 
methods rather than theory.
The reason why paint sticks to a substrate has to do with ADHESION (Did 
you search the archives with this keyword?). Here paint chemistry borders 
on the science of "glues", which has been amply reviewed by MADSCIentists, 
see e.g. at http://www.madsci.org/posts/archives/jul99/932360244.Ch.r.html
The references mentioned there are also useful for you.
Many of the POLYMER types used in adhesives (e.g. epoxy, urethane, 
acrylics) are also used in paints. There, they function as BINDERS, i.e. 
upon drying of the paint film, they prevent the powdery PIGMENTS from 
blowing away at the slightest breeze. Polymers also make the finished 
paint film look glossy, if desired. To be able to apply the paint in the 
first place, it has to be liquid: this is the task of the SOLVENT (except 
in the case of "powder coating", which is applied in a molten state).
These 3 components, pigment (white or colored), binder(s) and solvent, are 
present in every typical paint. In water-based paints, EMULSIONS are 
present in place of solutions. Historically, "drying oils" like linseed 
oil, had a double function of solvent and binder, since they transform 
into a crosslinked solid by oxydation in air. Artists' oil colors work 
that way. Other binders crosslink by radical polymerisation mechanisms 
(e.g. UV curing) or isocyanate reacting with hydroxyl groups to form 
urethane.
Paint films that are crosslinked cannot be dissolved (only swelled, ie. 
weakened) by purely physical means (solvents). If the bonds are ester or 
amide bonds, they can be hydrolysed e.g. by ammonia. I never heard of 
tomato as a PAINT REMOVER (see that keyword in the archives). The coating 
(LINER) on the inside of canned tomato cans certainly has to resist their 
attack... 
Best regards
Werner Sieber