The numbering system for polyurethane foam is very simple. The first two numbers describe the quality of the foam, and the second two numbers describe the density (ILD or indent load deflection) or firmness of the foam. The higher the first two numbers are, the better the foam quality is. The higher the second two numbers are, the greater the density of the foam, and the firmer the foam is. The first two numbers should be at least 22, with really high quality foam being 30 to 35. The second two numbers are as follows. 30 to 35 is medium, 40 to 55 is firm, 65 is extra firm, and 80 is hard. Back foam doesn't need to be really firm, or of very high quality because it gets very little wear. Seat foam, on the other hand should be higher quality and the density is a personal preference. 2235 would be good for back foam, and 3045 would be fine for seat foam. You can substitute a lower ILD foam for a higher one by making it thicker and wider and pulling it tighter. (BTW, the color of the foam does not mean anything, the only thing that matters is the numbering system).
There is a reason that seat foam is not 6" thick. The thicker it is, the more expensive it is. That's why they put the springs on the frame in the first place. The more open space and the less foam there is, the cheaper it is to make the seat. If the reat of the seat is O.K., why not just replace the back springs, which are the cheapest things in the entire seat system to replace? Back springs are usually 10 to 12 gauge (the higher the number, the weaker the spring is). Seat springs are 8 or 9 gauge.
You can put a piece of plywood on the seat back and build out with foam if you want to, but to reduce the weight you would add with a solid piece of plywood, and to make the seat back more comfortable , cut out the center of the plywood (leave a 3" border all the way around) and staple 2" wide elastic webbing criss-crossed onto the plywood which will replace the original springs, and still not be too stiff. Leaning up against a solid piece of plywood with foam over it would have no give and be uncomfortable after a while.
No one lives forever, the trick is creating something that will.
Last edited by DanTwoLakes; 04-07-2008 at 08:42 PM.