If the board has been throughly cleaned, you don't need to fear surface currents at 90% humidity. "Dominating surface currents" would be a fatal condition in my opinion, for high performance instruments a variation of 1% (< 1 Gohm surface resistance) or even less would be considered critical.
The problem is however to assure long term cleanness, also it may be difficult to assure condensation free operation. Thus conformal coating isn't a bad idea. A thorough analysis for critical points, where high impedance nodes come near to supply voltage nodes is also suggested.