Do I need to worry about if I grind in one spot 2 long and get small amounts of heat discoloration? Or since I will put body filler over it I shouldn't worry?

It isn't "good" but not a huge deal either. What you want to do is use nice sharp discs so it cuts instead of heating the metal. But that isn't always the answer, just one of the reasons for this. One thing is for sure , DO NOT apply the plastic filler to the burnt metal. You need to run over it with a nice sharpe disc as I mentioned before you apply the filler.
But really, cut with nice sharpe discs, keep it moving so you don't stay in one spot and you shouldn't have the problem.

and slow your grinder down, slow grinder speed and sharp abrasives will eliminate the heat. graemlins/thumbsup.gif

Another thing to look at is the hardness and grit of your grinding wheel. If you have a wheel that is too hard, it will not expose new, sharp grit which will cause the metal to burn. Also if the grit is small you may tend to apply more pressure and purn the metal.

Quote from metal supply shop:

" The hardness grade of a wheel is designated by a letter (A to Z, soft to hard). This grade represents the amount of bond contained in the wheel, which regulates the ability to hold the abrasive grain in the wheel. Normally, harder grades produce better surface quality, but provide smaller chip removal rates than softer grades."

Lastly, grind over as large an area as practical...if you're grinding 6" of weld, move back and forth over it all, this will allow the metal to cool between passes...don't attack a 1" strip at a time.

Ok, my body shop buddy told me to use a cutting wheel to grind the weld should I be using a actual metal grinding wheel?

Cutting wheel for grinding? I would suspect that by it's very nature a cutting wheel is going to be much harder bond than a grinding wheel. My .02-

I generally use a 4-1/2" angle grinder with a 24 Grit "S" hardness wheel. This may be designated at "A24S" where A= Aluminum oxide, 24= Grit, S= wheel bond on A-Z scale. This wheel will FLY through welds. It is soft enough to always be exposing nice sharp grits. The trade off is that the wheel doesn't last forever, but it's not too bad.If you plan to bondo the area, you now have some "tooth" for the bondo to stick.

If you are planning to work the metal out, I stop right at the surface (when the crown of the weld bead is gone) and switch to a fiber disk or flap wheel. Work through 36, 60, 120 grits with those and you will just about see yourself in the metal surface.

Thanks Corey, I just printed it and will pick some up tomorrow. Can you believe my buddy has a 2 year degree (just graduated 3 yrs. ago) in autobody and works at a shop that handles mostly high end cars? Evercoat recomends bare metal sanded with 80-180 grit for best adhesion on there rage gold so this makes alot more sense! graemlins/thumbsup.gif

OOPS...No more late night typing for me...Correction:

In my last post I mentioned an A-24-S wheel. I went and looked at the thing this morning. My "favorite" wheel is Walter Abrasives "A-24-HPS" I remembered the "S" but it seems as though they are using a non-standard designation system. There is a good web page to decode the wheel spec here:

http://www.radiac.com/pdf/spec.pdf

The walter abrasives page:

http://www.jwalterinc.com/examples/walter_us/w_docs/catalogue/Grinding.pdf

So Walter doesn't seem to specify exactly what hardness grade the wheel is. Sorry for any confusion. Either way, you may call up a local welding supply store and talk to the guys. They may have a wheel that is even better. The bottom line is that the closer the wheel is to an "A" (soft) wheel, the faster it will cut (but at the expense of wheel life)

Hope this helps.

Good information on abrasives here! Glad I learned something again. For rough grinding welding beads I have found nothing that beats 3M's 3inch X 1/4 grinding discs. These discs are exactly the same as the cutt off wheels they sell but are 1/4 of an inch thick. They allow you to remove bead heighth fast at a relatively slow rpm without any heat damage. And they last almost forever! I usually then finish up with a roloc disc in 36 or 50 grit and sometimes follow up with an 80grit D.A. Try the 1/4 inch thick discs.