When you use un-pure isopropyl you are using water too, 90% alcohol... 10% water

Water and oil won't mix, and maybe the water in the mix is "pushing" some oil from the epoxy resin out to the surface.

I guess you could try to clean that "grease" with the same isopropyl you used to get it thinned

Be gentle! or the isopropyl might remove the coating off your firewall [X(]

Saludos

Joseph