These devices have (or had) a conformal coating (at least the model 'D' shown in red there). "W" applied it (I'm "E", neither of us is involved in the business much this century). I'm pretty sure there was no baking involved - it was just sprayed on from an aerosol can as I recall.
Something like
this I think. Rapid are a pretty solid outfit; slightly less heavyweight than Farnell or RS but in that general area. Somewhat less of a toyshop than Maplin.