Friend I'm going with tells me onsen springs are very strict with tattoos and there's a big chance I'll be turned away, I have a tattoo right on my heart, 3 x 1.5 inches (around 7 x 3 cms), my friends tells me I can get a flesh colored patch and it should be fine but it depends on the onsen, and he doesn't know if you get those patches at the onsen itself or if you're supposed to bring it from elsewhere.

Also that some springs are strict enough they'll shoo you away regardless, and that if you can conceal your tattoo with your hand you can sneak in, but I'd rather be honest than risk getting kicked out. How much truth is there to all of this?