Buffing brass problem -Pitting/craters

[Tetsuaiga]

Would anyone be able to tell me why I'm getting this strange pitting when I've buffed brass?

I sanded starting at 80 up to 1000 alternating stroke direction 90 degrees each time. Then moved onto buffing, loose mop thing pink compound then rouge on different mop.

I buffed a brass rod using same steps and it seems to have come out better

Perhaps using a more solid felt wheel would be better?

 

[Rorschach]

Could be your coarse compound is too coarse, you could also have some grit stuck in it.

Mirror finish on properly sanded brass should be easy.

Normally I would say you didn't sand properly but since all the scratches follow the buffing direction that is unlikely. That being said if you have left deep scratches from the 80 grit, buffing in the same direction can wallow them out to look even worse.

 

[AndyT]

I think you are using too much compound. Those dark spots are lumps of it on the surface of the brass. Either carry on buffing or remove them with some thinners or white spirit on a rag.

 

[novocaine]

soft brass and going to slow with the loose mop thats too heavy. try a lighter compound (as said) on a clean softer mop and move the brass quicker.

 

[Rorschach]

Wait are you talking about the scratches or the black spots?

 

[novocaine]

could also be cold drawn brass, which isn't as easy to polish.

I'm taking about the pitting which is 1 of 2 things, either a heavy mop pulling out the soft spots or drawn grain that's harder than the rest and the brass hasn't been annealed.

 

[Rhyolith]

I solved that by dipping my cotton wheel in a bit of paraffin, then using it. Makes a mess (goggles!), but gets rid of all the excess in material in the mope which is definatly the cuase of the black sploges though not sure about the scratches.

 

[Sheffield Tony]

Noone seems to have asked what I'm wondering - what is it ? Is it just brass stock bar, or is it something that might have been cast ? If the latter, you could be just uncovering voids which were there all along.

 

[novocaine]

it's what I was alluding to with cold drawn brass that has a similar issue but the voids are elongated rather than round.

 

[chaoticbob]

Apologies if this is teaching granny, but...
To me the long scratches look typical of what happens when either the coarser grit scratch pattern hasn't been fully eliminated by the next grade up, or by poor hygiene when changing grits. It's necessary to be pretty obsessive about cleanliness - I do stuff which calls for a mirror finish on brass and wash the parts in solvent between grits and either wash my hands or change gloves if there's any sign of contamination. Might sound OTT, but the tiniest bit of coarse grit left on the work can wreak havoc when it comes to the final polish. As with all finishing (I've found this with wood as well) the final stage really shows up any imperfections in preparation.
After experimenting with various compounds I've ended up using Autosol sparingly applied to a loose leaf mop for the final polish on brass - works for me.
Rob.

 

[wallace]

I think it is the composition of the brass, Have you tried sanding through the grits to maybe 1200 and then just polish with a rag and a liquid polish like t-cut or a paste like autosol, doing it hand might work better.
I've never come across the affect like in your pic, you can always tell if you've been sloppy and rushed a grit, but the mop will usually remove any trace.

 

[Tetsuaiga]

Thanks for the replies.

I tried using a felt wheel with rouge on, that cleared up all the pits. It must be something to do with either the mop or the compound on it. I do just use a drill press too, with the felt wheel I did use my wood lathe but doubt that makes much difference.

I have some autosol so might give that a try too.

The brass is just stuff off ebay, I think it's quite common, it a little sticker saying made in America some state.

 

[sunnybob]

80 grit is incredibly violent for brass. It would take forever to remove scratches that deep.

If doing this by hand I use 240 as starter, go to 360, then 0000 steel wool, and then solvol autosol.

If using a polishing wheel start with brown paste and move onto blue /light green, then solvol before moving to the lambs wool polishing mop, making sure the piece keeps moving so it doesnt overheat.

If those splodges are lumps, it shows the paste got overheated. They can be scraped off.
If they are hollows, its poor brass and needs a lot more polishing, maybe even filing.

 

[Tetsuaiga]

I used 80 grit on a drum sander because it came with sloping chamfered edges and I needed it all flat.

I pretty much just treated it like wood though going up grits so wasn't that bad.

I had the same problem again today but tried increasing the pressure and it completely changed how the buffing was working. I think the low pressure was definitely part of the issue.