:My concern with the plywood for stattions
: is the possible void that I will encounter.

These are building forms, not fine furniture. The occasional void in absolutely no impediment to stapling. Odds are that your strips won't hit the voids, and if they do you can aim the staple left or right of the void.

:Also the cuts on the plywood are likely to be ragged.

Sand them. Aggressively. You'll be doing enough sanding later on, so you might as well get used to doing some now. You DO like sanding, don't you?

The MDF I looked at is 1/2" and although it
: is heavier than Hell, looks like it will resist warping and is solid thru.
: Regarding the staples, how does MDF hold staples. I am going to try to go
: stapleless, but may have to revert.

If you can lay out your forms carefully, there is very little difference in price of materials for the forms. 3/4 inch B-C plywood by me is about $7 more per sheet than the cheapest alternatives you have mentioned. if you can get all your forms out of one or two sheets, then the difference is at worst, $14. MDF holds staples fairly well when driven in the edge, but the surface is very hard, and your stapler won't like it if you need to tack something to the face, rather than the edge, of the form. Same for particle board, which hasn't been mentioned, but some people use. The OSB I keep seeing is 7/16ths thick. I'm certain there is thicker stuff around, but it just doesn't jump out at me when. the 7/16ths is going to be on the thin side of useable materials. Yo cna get it to hold a staple acceptably, but you need a better aim to get the staple into it.

Heavy forms place more stress on your strongback so you'll need more wood for more legs to brace it at closer intervals so it doesn't sag. If you buy cheap, heavy materials for your forms, you'll need to buy more wood for those extra legs. Then suddenly things aren't so cheap.

PGJ