Heyas,
http://www.uboataces.com/sonar-coating.shtml
I believe it is in the game, though I can't remember what year it comes up. I recall that it only has an effect of around 10% decreased chance of detection though. GWX manual shows the upgrade becoming available in '41 (alberich) and two others following it in '43 and '44, though I don't know anything about those because '41 is always my graveyard :P
::Edit - Beaten, didn't see the above posts
That's what happens when you open 20 tabs at once and read through them over time.