yeah it is oil
The paint may not attach to that spot.
Ok, well I wasn't going to paint the cardboard anyway. I put the fiberglass resin and mat over it and it came out great . No weak spots. The "oil" did not effect anything. I drimmeled out the stuff I didn't need and all I have to do is paint it. The foam I used is non fire retardent. I will have pics up later tonight once I'm done.lol.....just to confirm...it IS oil....and it may effect your paintjob......and if you are coating it with resin, the resin might not be able to penetrate the cardboard and may result in a weak spot that will crack easily.
Another thing to be carefull of is using fire retardent upholstery foam next to your skin as the fire retardent stuff tends to be cyanide based and your skin might absorb it........so best to try and cover it with something......or make sure you use the non fire retardent stuff.
Good luck with your project.