"Tim W" wrote in message

That may be true today, but if by hardboard the OP meant tempered
masonite, it used to be available smooth on both sides.

Yes I think they call it masonite in america. I stand corrected. I have
never seen hardboard smooth both sides, but plenty of 3mm mdf.

Masonite is the trade name of hardboard made by the Masonite Corporation,
founded by William Mason. They no longer make that product, but they do
make doors.