I've never tried this but I think Wite-Out
would work well. What I'd try is painting the surfaces of the pieces with it. If thick structures of the material dry on the surface of the part you should be able to buff them out with a light hydrocarbon solvent. The idea is that the wite-out + solvent will do a decent job of penetrating the porous material and you can rub the excess wite-out off the surface of the part.
I'm having trouble finding technical information about the modern "safe" correction fluids but some might be water based. It's worth a try even if you can't find a non-water based one. In that case just use water as the solvent to get the excess dried correction fluid off of the surface of the part.