This began as a reponse for JClay but I thought it was worth making general because it governs the interaction between an applied liquid (like paints or adhesives, which includes brazing alloys*) and an existing solid (the thing being painted or adhered to (or brazed)).

Rule 1. It won't stick to what it won't wet.

This comes under "that which is retrospectively obvious" but it bears repeating : since adhesion relies on an interaction between the adhesive and the adherend, you can't make anything stick to a surface it can't wet.

This begs the question of what governs surface wetting.

Rule 2. A high energy liquid won't wet a low energy solid.

The liquid has to lose energy by wetting the surface: otherwise the energetically favourable state is for the liquid to stick to itself.

So what is this surface energy?

A surface is where one material stops and another starts. Normally the forces on the atoms / molecules in the material are in balance with the other atoms / molecules surrounding them but logically at the surface they are only surrounded on one side so the forces are unbalanced. The energy it would take to rebalance the forces** is the surface energy.

Rule 3. Surface energy depends on material type.

The forces fall into two basic groups: dispersive forces due to things wiggling about (AKA Van der Waals interactions) and polar forces due to the interactions between polar groups. Both depend on the properties of the atoms / molecules in the material, logically non-polar materials have only the dispersive forces and polar materials have both dispersive and polar forces.

Since dispersive forces are weak, non polar materials have low surface energies. Many polymers, for instance, are either non polar or weakly polar so they have low surface energies. This makes them hard to wet and therefore hard to stick to.

Polar forces are stronger so polar materials generally have higher surface energies, making them easier to wet.

There's a good guide to surface energies here but I'll summarise, all figures in mJ / m^2

Low energy polymers (PTFE, polythene) 20 -30
Medium energy polymers (PVC, epoxy, Polyurethane) 40 -60
Ceramics 100 - 500
Metals 1000+

Next post I'll give some examples of why this matters when making bicycles

* Brazing alloy is just glorified hot melt glue.

** The forces get weaker with distance. The energy is the integral of the force with respect to distance all the way out until the force is effectively zero.