what's the best to use to grind welds down, coarse grit sandpaper or a grinding disc? i don't want to get the metal too hot and warp.

I use a 36 grit sanding disk on my grinder.

I've seen small air powered belt sanders for this porpouos, they be goood

I am just learning how to weld, so needless to say, my trusty ole grinder is coming in quite handy. The most amazing thing I've found to smooth out the rough welds is one of the flap wheels that fit on the 4.5" grinder I have. Not the sandpaper looking one, but thick like a grinding disc, with little flaps of 36 grit paper stuck to the base. They work amazing, and you can blend as the edges roll over a bit. I use the reg grinding disc for somethings, but most of the time the flap wheel.

Something similar to this:

I tend to use a regular wheel to take down the big spots and then switch to a flap wheel for the second stage. They sure do work slick and they actually last better than I figured they would.

Good point Big Jim, I was actually thinking that it would be nice to have a cheap grinder as a spare (one of those $15 jobies), and have a grinding wheel on one, and the flap on the other.

My welds today were...ahem.. .I actually did switch to the hard wheel to take down the high spots, then reset the grinder back over to the flap to blend it in (they flaps are just a bit bit pricier, but more importantly today , not as many in stock in a small town. Might as well save them for the blends).

sorry, guess i must have timed out...but that last post was mine.

I use a grinding stone. Very little heat is exchanged and a stone lasts a very long time. A stone will grind it so smooth, it'll look like you didn't even weld there.

I bought a box of 10 flaps off eBay a while back for about 1/2 of list including the shipping. I live in a big city but eBay can be a great resource for the people a ways 'out in the boonies'. I really like the way flaps seem to work on the high spots and go really easy on the areas that are already flat.