I’ve been having interesting problems getting oil-based gloss paint to cure/dry properly on some parts of my boat - for example, paint applied on bare or primed bare wood is just fine, but on glue joints (regardless of whether they’re epoxy or polyurethane glue, or supposedly overpaintable acrylic sealer), the paint remains tacky for an unreasonably long time.
However, when the same paint is applied over the top of an existing gloss painted surface, it dries quite normally - so it can’t just be the absorbency of the surface.
Paint drips that got onto my rubber gloves don’t dry at all ever - paint drips that got onto a smooth plastic bottle dried normally, as did paint residue on the outside of the can.
So what’s actually going on here? Why would an oil-based paint dry on some surfaces, but not others?
From what you describe, the problem may not even be that it’s oil-based.
Oil or water are only the solvent. In order to dry, two things must happen to the paint:
Its solvent evaporates. This should not depend on the substrate (unless you want to toss capilarity into the mix, but once the substrate is completely covered, this factor disappears).
It bonds with itself and with the substrate. This does depend on the substrate. You already know that a waxy crayon will work better on paper than on glass; same as the waxy crayon bonds better with paper than with glass, the solids in the paint you’re using bond better with some substrates (the plastic in the bottle, wood, previous paint) than with others (glue, your gloves - looks like it doesn’t like “rubber-like” substrates).
Oil-based, water-based and no-solvent paints contain solids of a different chemical nature, which may behave differently with the same substrate. But if you had, say, an oil paint where the solids were polybutyrene and a water paint where the solids were polybutyrene, both would behave the same way with the same substrates. (I worked in a paint factory and we didn’t use butyrene as a monomer, I just wanted a word to use).
Yes, that’s right - the paint on my gloves is still sticky a week after the paint on the wood (and the residue on the brush in my waste bag, and the drips on the floor and elsewhere) has dried completely.
The paint on the glued areas of wood took approximately three times to dry as elsewhere (OK, I can understand that the wood might wick away and disperse the solvents), but the second coat of paint (over a coat of cured gloss that feels smooth and inabsorbent) dried quickly and evenly.
Yes, but the failure to dry is linked to the paint reacting with itself and with the substrate. The whole volume has to react. These reactions actually begin at what could be called “the surfaces of the paint layer” - both inner (in contact with the substrate) and outer. If the inner surface isn’t initiating (nucleation problem is one of the possible causes for this, another would be a chemical incompatibility which would lead to adhesion problems), then the whole thing moves a lot slower.
I think you just answered your own question.
When applied to bare wood, the solvent has to escape for the paint to dry. Some of it evaporates into the air, some is wicked into the wood. Painting over glue, or your glove, and no wicking occurs. The paint skins over as the solvent in the top part of the paint evaporates, and this traps the rest of the solvent below this skin. With the skin in place drying continues but at a much slower pace.
When you put on a second coat, you are dealing with a homogeneous surface. Unlike the first coat everywhere you put paint you are painting over the first coat. In this case some of the solvent goes into the first coat to try and dissolve it, and the rest goes into the air. As a result you have a normal drying time all over the boat.