: I can buy 4x8 sheets of 3/4" MDF for $20
: locally, and was wondering if this is a reasonable material to use for
: forms. It seems everyone talks about using 1/2" plywood? Form cost
: seems small compared to everything else involved, so I guess a few dollars
: either way for a better material isn't going to break the project.

No sense wasting money, either.

The thinner material is cheaper and for our purposes just as good. If you are going with the staple approach you just need to have a slightly better aim if you are trying to get the staple through the strip and into the form. Long before the 10th strip your aim will be just fine. If you are up for a challenge -- go for 7/16th material.

Heavier forms put more weight on the strongback, so you end up building a stronger ( and more expensive) strongback to avoid sag. So, with lighter forms you can get away with a cheaper strongback. If you are looking at using a 2x4 for an internal "strongback" then you should definitely consider the 1/2 inch material.

Wood is not even a necessity. People have used corrugated cardboard (laminated to be several layers thicker than what you usually see on your average cardboard box) and polystyrene foam insulation board for forms. With a hot wire cutter you can go through that foam easily, with little mess, and no sharp or expensive tools. Staples won't hold in it, though.

Hope this helps.

PGJ