Brian Gaff used his keyboard to write :
Really, I know some car body filler gets hot as it cures, is this what you
mean.

No, it is the chemical hardener in the filler which attacks the
polystyrene, it basically turns it into slime according to what I have
read. I just needed some advice on whether just keeping the two
separated was enough, or whether one needed to be sealed from the
other.

I saw a video of such a repair and they used some substance to seal
over the polystyrene, before applying the filler.

--
Regards,
Harry (M1BYT) (L)
http://www.ukradioamateur.co.uk