How fast do you move the grinder when grinding glass for stringers? I move pretty fast to keep the heat down. Some else says to move slowly like grinding wood. What say you?

IMO it's a combination of both speed AND how much pressure you apply . One thing for sure , the more you do it the better touch you develop.
If you're grinding to completely remove glass that's one thing . If you're grinding to fair for a repair or another layer of glass , then that's another. If you're doing a large project you'll be an expert when you're done.

Just a side note .. when I did my last major project I had a disc shatter unexpectedly . Because I was working on a flat surface at waist level the largest piece of shrapnel hit me in the stomach . All I had on (from the waist up) was a T shirt. I thought I had been shot and was fully expecting to find a hole in my gut.

had a roloc loose its arbor at full zing, went through my shirt and took a good chunk out of my side, still have the scar. thought I got shot as well man that sucker hurt.

grinding should be a fluid even motion. Speed, pressure, direction etc all depend on the surface and what your grinding. Grinding for fast material removal and gringing for a proper mechanical bond are different techniques, In general though fluid even movmants with even pressure are gonna be a good start, I actually call it buffing with a grinder. When a disk is new it also needs less pressure and you can let the grit do its job, as it breaks down more pressure is necessary.

when doing heavy knockdown of material do you guys see any benefit to larger (9 inch?) grinders? I was thinking about this when i was doing my boat but i didnt feel like buying one cause they are pretty expensive

I have used a 4" 36 grit flap disc on a 4" electric angle grinder before.. you can make some serious progress with that setup.

Take heed here that is a bad @@s combo, i was so impressed with the ability to cut hard fast and controlled i cut down my cedar deck in a few hour's rather a large deck at that. It was the talk of the neighborhood for a while...my next door neighbor didnt know wether to ***** about the cloud of cedar drifting or how fast it cut and got down to new wood....amazing..

yeah for big flat stuff, I use them all the time. I like Norton Blue disks, seem to last the best.