How to treat poly hives
Most poly hives come in the same stark white of the EPS used in packaging. If only for this reason poly hives are normally painted, but there are also practical reasons for painting a poly hive externally - the paint will protect against UV light, which will degrade the surface over time, it will also seal the surface against moisture giving protection against surface frost damage and discourage the growth of algae which can attract slugs and snails, whose rasping tongues will damage the surface.
The usual recommendation is to paint the floors of poly hives on all surfaces but the bodies only externally. However, poly feeders need to be painted internally. This seals the surface and allows easier cleaning, otherwise sugar syrup can leach into the material - there are tiny voids between the expanded beads which make up the matrix.
Almost any sort of paint can be used to paint a poly hive - even the aerosol paints used for spraying car bodies have been successfully used, although the presence of solvents in these paints makes it a slightly riskier operation.
Gloss paints, emulsion and acrylic outdoor masonry paint have all been recommended by those with experience of poly hives. The main requirement is paint with good adhesion and a degree of flexibility. For larger quantities of hives spray painting is by far the easiest but a brush or roller works equally well, but is more time consuming.
I get around the country a lot, visiting BKAs, beekeepers and apiaries. I see a lot of polyhives and in many cases they show quite severe signs of degradation, often when painted. Certainly surfaces often crumble badly, sometimes when they have clearly been painted for some time, so not painted after the degradation has started.
At Wisborough Green BKA teaching apiary we have a dozen polynucs from two different manufacturers. I have painted them with "Sandtex Ultra Smooth" masonry paint, giving two coats with a brush, working it in well with circular movements, not simply slapping it on in straight lines. This should work the paint into the surface, so preventing water fracturing the surface when it freezes. I found this paint went on quite well, but I had to thin it down with water a little to make it spread better.
I bought our paint in a hardware store at below half price, because it was a discontinued colour.