Tar+rain=Worst thing ever!!
I'm not going to tell you where to get it from, since I don't want to be blamed since road tax never seems to want to fix the lovely hole you got it from
There's a clue in there
I'd normally advise people not to use tar though, since the slightest bit of moisture and you've had it.. Do you have access to an angle grinder? Rim grinds are a hell of lot better IMO, since you can ride in the wet with no problem..
You also have the option to still use tar if your brake goes poop (no-one knows why, buy it happens to all of us)
Whereas more tar on top of tar just makes it worse:/
I carry tar with me all the time for emergencies