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Masks do not affect blending modes or else they would be uselss. Whatever you do has another cause. Could simply be that your graphics card is not up to speed and can't handle certain ttransparency operations. turn off OpenGL usage in teh comp viewer or the prefs even.
thanks for feedback. Sorry I forgot to mention the the illustration layers with multiply transfer mode were in the illustration precomp.
When I masked this illustration precomp in the master comp the white background of the illustration layers became visible.
I resolved this by putting the solid coloured background from the master comp into the bottom of the Illustration precomp, then put the illustration precomp at the bottom of the timeline in the master comp.
I could then mask the illustration precomp and the transfer modes were preserved.
Hope that all makes sense