<< I primed some hardiboard last fall and let it go through the winter. There
is some signs of mold >>
How do you know its mold and not just common dirt, grunge, urban smog deposits,
or some such?
<< What is best procedure before priming and painting? >>
Celebrate! Now you've got an excuse to buy the hottest yuppie toy, a power
But, if you have proven that it is in fact a mold growth, there are the usual
remedies mentioned exhaustively in this newsgroup. For different approach I
would clean it (scrub brush, power washer, whatever) and, if the area wasn't
obscenely large, use a heat gun to thoroughly dry it, then continue the heat
application to a fairly high surface temperature, say 270 F (which ought to
kill mold spores dead as Hades), and as soon as it cooled to ambient, slap on a
coat of primer, maybe with a mildewcide added.
Meanwhile, discussing the problem with the hardiboard customer service people
might elicit some helpful information. Perhaps the paint folks, too. Good luck.
Heat to 270 ? bubble the primer and ruin it more likely.