This is a portion of a post of mine that I posted in discussion about these super long blocks on another forum. (click here for the thread at Team Chevelle)
These are my thoughts on these monster blocks, am I off base, do they really make a difference? The answer from Durablock on the original posters questions about wrinkling paper was particularly interesting and seems to support me.
I don't use anything any longer than a 17" "Long board". This is how I see it, and yes I can get a car VERY straight, if the long board is ran across that door it is going to flatten it just as good as a 30" block. I mean, if there is a low spot in the door that is wider than the 17" than you have other problems, more serious problems that no long block is going to fix.
Let me put it this way, those super long blocks have only been around, what maybe ten or fifteen years? When I first started in this business there was no paper to even go on these long blocks, they simply didn't exist. So were there no straight cars? Of course not, there were cars just as straight as now.
The thing is, NO Panel is "flat", none, period. So, what good is a block that when laid on the panel is up off panel at each end? Honestly, how could that block that is not touching more than a foot or two at a time block the panel any straighter than a block that was that foot or two? It can't, that is the fact.
The curveable blocks don't CUT the shape because it doesn't match the shape either. I mean, it will match a PORTION of the panel, but again, not the whole thing. So if you bend it into shape it will only be good over a small area. And if it flexes to fit the shape, it won't be "flat" enough to CUT the high spot "flat", I just don't get it. I have tried them, I have used everything you could imagine at one time or another to shape things. When it comes to concaves, I have used rolls of paper towels with paper on them, every friggen size rad and heater hose you can get. I have bought wood blocks cut to the shape of body lines, half round "blocks" and so on. But with "flat" panels, good old rubber blocks, long and short and long boards are pretty hard to beat.
Durablock round, and the long rectangular ones are super and I use them. But the super long monsters, I just never found a need. I may be working on a car in the future where I run out and get one because I decide on that project I need a super long block and really like it, I don't know. But as of now, I have lived without it.