: Thank you Mike,
: I sanded and coated once... Looks much better already! BTW, for the
: sake of any future builders the Rustoleum professional flat
: black has limestone in the formula and I think might be the
: culprit in the cracking finish.
: Cheers!

H Tavo,
I would use satin finish rather than matt/flat, it stays more flexible than the matt and gives some but not too much shine. It doesn't get quite so hot under the deck as well. Though that's not a problem here with our cloudy weather this year. :)
But multiple thin coats tend to crack much less compared to heavier coats regardless of matt/satin type of paint.

Mike Savage
South West Cork