Seeing weld marks from the other side?

[Scratch]

I just finished this "overlander rack" for a customer. It bolts to the top of his pickup truck bed sides, and is basically just a frame to mount a big tent to. It turned out well but one part of it really bothers me...
I did some welds on the back and you can still see them on the front.


I'm ok with them on this job since you really need to be in the perfect light to see them, they're not that bad, (the pic looks worse than they are) and they'll be covered up a lot of the time, but I want to know how to get rid of them for next time.

The sheet steel in this pic is 1/8" and I know if I used thicker steel, it will help, but what do I do the times I need to use thinner steel like this? I did softly run a flap disc over the front where these were and that helped, but if I did too much grinding, the mill scale would be removed and then I'd start having other issues like grind marks. I thought I removed it and couldn't see or feel it, but once the paint got on there, it reared its ugly head.

I'm assuming the solution is to do a nice hard grind and make sure I remove all of the HAZ, then go higher and higher grit sanding out the grinder marks, and feathering in the mill scale edge, but is there another tool or solution I'm not thinking of?

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[SeanP]

Maybe add a couple of vertical slots and plug weld onto the box section and not weld at all on the other side?
Might be easier sanded flush that way.

[adbuch]

You could use cold rolled so you don't have to deal with the mill scale. I buy 11 ga. CR steel in 4 x 8 sheets and cut down to the sizes I need.

Another thing to consider is that any time you do a fillet weld, there will usually be shrinkage which will pull the parts together. So if the parts started off with an included angle of 90 degrees, after welding the angle will be less than 90 degrees. This would tend to bow your 1/8" plate, and after straightening - you still may see some evidence of the "bend" - which could tend to accentuate your weld bead distortion.

If it were me, I think I would do as suggested above and cut some slots (I would be more inclined to use round holes) and do some rosette welds. You can grind these smooth (I like to use "flap discs" - #40, #80, #120 - in that order). Then maybe apply a little texture (spray can) to match the existing scale on the surface of the hot rolled steel and then paint.
David

[plasmanewbie]

Nice rack you've got there!

I had the same problem with a project but it had to be perfect. Ended up grinding it flat and using 3M autobody high build primer to fill the scratches. Misted a black trace coat on primer and sanded with 400 until the scratches were gone then painted.

[robertspark]

flap disc?

[adbuch]

Flap Disc.jpg

[robertspark]

yup... always works well for me if I need to tidy up any welds.

[Scratch]

Yes, I use flap discs. When I grind, I almost exclusively use flap discs.