Even if the electromagnetic material properties of the paint were drastically different than that of air, the few microns of coating won't be even measurable. (Unless that coating really is conductive paint)
Another thing: You've bought an antenna. Building exact antennas is hard. If you worry about the effect of 30 µm of paint coat, then you'd also have to worry about the effect of temperature, shrinking and expanding the PVC pipe. Or the fact that no one cuts conductors for antennas with a 30 µm accuracy.
Why are you worrying? It's not like your antenna was exact to a high degree anyway. In fact, measurement antennas whose directivity is known to less than a percent are expensive. Price of a small car for antenna expensive. With a dipole variant antenna like yours, you'd be happy if the actual center frequency (ie. the point of highest matching to the nominal impedance) everything is let's say with +- 10% of what you expect. It won't work worse if a slight wind blows, but that would flex the PVC much more than a coating would add.