Pits in finished stainless

[badav]

Hi all, i am polishing up some old stainless steel moulds. I start with a sisal/emery combo and then finish with spiral sewn/green & loose/white.

The piece shines up almost mirror after cutting with spiral sewn/green however after this step there are also many small pits left in the piece. They are only visible when looking up close.

Does anyone know what causes this?? Is it excessive heat?? I find that if i don't apply enough pressure (which heats the piece) then the buff does not cut well enough.

I have also found that sometimes when dry sanding with 240 grit it can leave bigger pits sometimes.(which are very hard to get out).

So what causes this to happen?

I took a photo which shows what is happening:

[mpierich]

That looks wierd. You mention 240 grit. Is that the highest you go with sanding? If so, you should be going to at least 320 or 400 grit before beginning polishing. Does the surface look like that before you do anything, or does it only appear after you start polishing?

It almost looks like softer aluminum would after using a heavy sisal wheel on it.

[badav]

The surface doesnt look like that before polishing.

I start with 240, then go to 320 then 400 before i hit the buffs.

The pits only appear after i have cut the piece with a spiral sewn buff with green compound on it.

[mpierich]

Originally Posted by badav

The surface doesnt look like that before polishing.

I start with 240, then go to 320 then 400 before i hit the buffs.

The pits only appear after i have cut the piece with a spiral sewn buff with green compound on it.

Only thing I can think of is to make sure on your sanding that you're going both directions. In other words, if you go lengthwise on a tube (for instance) the first time, go across the length as well at a 90 degree angle to the first sanding. Use a bright light and watch to see if anything shows up that isn't going the direction of the 2nd sanding. That's good practice with all the steps, tho it's not always possible on all shapes.

[badav]

hmmm. i am practising 90 degree sanding. although it is a pain and very time consuming to sand this piece vertically.

stainless is supposed to be a hard metal but something i am doing is eating it up bad

apart from the pits there are also some less noticable gouge marks which can be seen near the top of the photo too.

[Rasper]

Do you know for certain that they are stainless? Stainless is hard. To cut pits like that in stainless would take more than a cloth buffing wheel.

It looks like you have a metal which has little pockets of a softer metal embedded throughout, possibly having precipitated out from being heated. For instance, the chromium will precipitate out of stainless if it is heated to around a dull red and held there for more than two or three minutes.

Richard

[jrow]

badav,

It looks like on the left side of your picture that this area is not polished, correct?
Anyway try using some white compound/sewn wheel, without sanding and see if that will polish it. Make sure you clean your buff often. The heat caused by the pressure that you put on the wheel is what gives you your shine. How thick is that piece you are trying to buff?

John

[mpierich]

Originally Posted by Rasper

Do you know for certain that they are stainless? Stainless is hard. To cut pits like that in stainless would take more than a cloth buffing wheel.

It looks like you have a metal which has little pockets of a softer metal embedded throughout, possibly having precipitated out from being heated. For instance, the chromium will precipitate out of stainless if it is heated to around a dull red and held there for more than two or three minutes.

Richard

He does say they are old...my understanding is that when stainless first came out it was not a very good material. Perhaps it's the quality of the stainless...?

Maybe if he skipped the aggressive sisal wheel it wouldn't tear it up.

[badav]

The moulds are around 33 years old. It is auto trim. I am sure it is stainless steel.

Well i started from scratch again sanded with 240,320 then 400. I tried one piece with the sisal buff and one without using the sisal buff.

If anything the pits come up worse if i skip using the sisal buff. However if i don't use the sisal buff the gouge marks which are perpendicular to them do not come up.

[baz]

i have done mouldings on cars that were older then this and the stainless came out ok ,be careful on old cars you could have on the same cars windshield mouldings in stainless and side mouldings or hood mouldings made out of anodized aluminium .
looking at the picture it does not look like pits but more like sanding marks or even grooves left by a wire wheel or something similar .
i would try JROW s suggestion and see what happens if you don t sand before buffing , first with sewn wheel , if no grooves show then try again no sanding but with sisal wheel then you will probably see if problem came from sanding or the sisal wheel .
but to be honest with you . if they re stainless i don t think tht the sanding or the sisal wheel did these marks there might be a part of the puzzle we don t have