After welding in a patch panel, at what point am I doing more damage than good when grinding the weld down? Does it have to be completely smooth before applying filler?

Thanks

I would grind just a little more or else you'll need to use a lot of filler to level the whole area. It wouldn't hurt to weld up those large gaps. Then grind down and it will look good.
Are you using one those flap disks? Those are very good for knocking down and smoothing the welds.

I will grind a little more on that section. Here is a pic of the other half. I’m just afraid I may be removing to much good metal on the surrounding areas. Yes, I’m using a flap disk after initial grinding with an angle grinder.

Flap disk ... NO, NO,NO. The weld is usually harder than the surrounding metal. The flap disk has too much flex and will remove the softer surrounding metal more than the weld - so you end up with thinner base metal and thicker weld. To grind welds you want to use a solid composite disk OR if you're using an air angle grinder you can use the small 2" or 3" disks on the appropriate mandrel.

Ok, thanks. That’s what I was afraid of. I will continue using the angle grinder with a 2” disc. Do the welds all need to be smooth and blended in before filler, or are small imperfects ok?

You really should be using sanding discs like the ones below either 24 or 36 grit. As stated, you have to be careful not to take off the parent metal. These discs allow you good control and work great. Right angle grinder/sander and you vary rpm as necessary and always keep it moving.

You use backing pads like the ones shown below. As the disc wears, you go to a smaller backing disc and trim the edges of the disc with a pair of tin snips - does not need to be perfect.

https://www.autobodytoolmart.com/product/3-pc-phenolic-backing-plate-set

Sanding Discs

That is the technique I used in this guard on the edges - there is no plastic in that corner.

Any weld high spots will stick out like a sore thumb. Once you get all the welds pretty much even with the parent material, you do just a touch grind/sand on the whole surface.

For small things I 've used a Dremel with the brown cutting discs stacked together to knock down weld beads. The discs are brittle so you can't use just one and expect to apply any force to it. Not super fast, but this method does allow you to have nice control over grinding down the tops of the welds without touching the surrounding material.

What panel is that? Looks like a trunk drop-down. If that weld area is "hidden", more important to fill gaps then to flat welds.

Nice work on that cover Markz528. I will need a lot more practice before I’m that good.

Thanks for all of the good tips everyone.

It is the front lower edge of a 65 dodge D200 hood. Not really hidden.

I will try some of these tips to see if I can get this project finished up.

How far to grind? Depends on how good your repair is after welding. If there is some warpage and the weld isn't higher than the rest of the area by more than .060" your probably very close. Grinding to thin just for sake of appearance now could cause issues later. Use a straightedge and check for high spots. If it's low leave it.Make sure there are no pin holes or seal it from behind. Personally a cut off wheel with good hand movement will knock the weld down quicker with less surrounding damage to the sheet metal than a disc. then finish with a hard backed disc. We are hobbyist here. That means as we learn our patches will not be like someone who has done it for years every day. The longer you do it, the better you will get. I did these awhile back. I've also done some when learning that weren't as nice as the ones you pictured. You're off to a good start.
Doug

I use a light in behind the welded area (if you can) after grinding, to find any pin holes.

I have used the light trick before trying to find pin holes. I have used a cutoff wheel to grind the tops of the welds down too.

I have a build thread going over on Sweptline.org if anyone wants to check it out.

http://www.sweptline.org/forums/vie...amp;sid=af1f95ff279aecb856abebe1b549685a

Thanks.

There is nothing wrong with using a flap disk. If you angle the flap disk just on the welds that will work fine and then finish as someone stated with an 90 degree angle tool with the the disks that were pictured.

Knock yourself out bud - but I can pretty much guaranty you won't find flap disks in a professional body shop. And it isn't because they can't afford them.

Thanks for your input. Didn't want to get into a debate here. Just giving a suggestion on this topic as others have.

They do serve a purpose. Nice to use for a smoother finish. Takes off some grinding marks a cutoff wheel may leave. And they do come in various grits.

Markz28 thanks for the tip about trimming the sanding discs with a tin snips. I would have never thought of that but it does work great at keeping the disc grinding good on the outer edge.

I guess if I was that close I would just finish it off with a file.

Thinking about one of these:

https://youtu.be/fPDbu_T0kL0