A belt sander is easier on the heat treatment of a knife than you might expect. When I use a 36-inch belt the heat going into the belt is distributed over 36 inches. This 36 inches of belt is running over metal rollers and running around a track in free air. The net result is that the belt hardly warms up at all. As you grind down the steel you release heat from ripping apart the bonds that hold the steel together. Some of the heat is transferred into the blade while some is transferred to the metal debris removed from the edge and some is transferred into the belt. I would roughly guess that 1/3 of the heat generated goes into the blade.
What you want to do is to work in a way that heat does not build up in the thinnest part of the edge faster than it conducts away to the air, belt, and remainder of the blade. If you generate the heat too fast the temperature right at the edge will climb into the range of steel tempering temperatures. For simple carbon steel this is up above 300 degrees F and for stainless steels it is above 500 degrees. When you are close to those temperatures heat transfer into the 100 degree belt and the 75 degree air is pretty fast. The belt generates a pretty good breeze as it speeds along. If you keep your fingers in contact with the blade and grind intermittently (grind a little rest a little) you will find that the blade gets uncomfortable to touch if the edge gets close to that temperature range. Even at those temperatures heat treatment degradation takes minutes rather than seconds to occur. If you work too fast you could get much higher than those temperatures before you felt the heat in your fingers.
I like to grind edge-first. My sander belt is running downwards on the side where I do my grinding. I grind with the edge up. I do this for a couple of reasons. One reason relates to heat. I figure that as belt grit gouges out tiny furrows the grit and metal gets progressively hotter at the final end of the furrow than it is at the beginning of the furrow. The grinding transfers heat more to the downstream part of the grind than to the upstream part. When I grind edge-up I transfer more heat to the thicker part of the blade than I do to the apex of the edge. This helps to protect the heat treatment at the edge (which is the most vulnerable region). I also grind intermittently and hold my blade close to the belt when I release grinding pressure. This leaves the edge in the air draft from the belt between grinding intervals.
As an aside I want to note that for a given amount of material removed you generate more heat with a fine belt than you do with a coarse belt. The overall quantity of heat generated by removing material depends on how many metallic bonds that you break. If you pulverize metal into a fine dust you generate more heat than grinding it into larger granules. So if you want to remove a lot of material use a coarser belt, BUT you need to be careful since you will remove material faster. You need to control your maximum rate of heat generation. You can remove material faster and stay within your heat limit with a coarser belt. You just have to be careful since the coarser belt enables you to hog off material at a very high rate and generate heat very fast. You still need to grind intermittently and keep your finger in touch with the blade, but the work will go faster.
