Hi Nige,
As the MM is ABS, some (black) scrap solved in MEK (or Acetone) will give you a goo, that will fill those gaps very good, you can determen how runny the goo will be by the amount of MEK added.
As it's a (potent) solvent, fill the gaps with small layers and let the solvent evaporate before adding the next layer.
Don't allow 'pools' to form, it'll deform an ABS hull (don't ask how I know that...).
Regards, Jan.