Ray J 's doesn't skimp out on the SKIMS jokes on our "TMZ After Dark" tour ... going into a full facetious meltdown when seeing Kim Kardashian 's L.A. store.

The rapper took the helm of the tour bus with DJ Mike T of VOX DJs backing him up Saturday night ... and, he cracked up the riders when passing the store -- screaming and curling up into a ball while making a pun about SKIMS. TMZ.com

As you know ... Ray J and Kim K used to date -- he's the dude from the now infamous sex tape -- and, though he's far removed from the relationship, it seems he still knows how to use his ex for an extra wisecrack here and there.

Ray J also loves to get to know the riders on our tour a bit ... though he may want to stay far away from one he met this weekend -- who showed off a tattoo of notor

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