Mirror finish for mild steel...?

[The Z'eer]

Front and rear exhaust sections are off the car at the moment, and as such I figured I'd remove corrosion and polish the pipework to a mirror finish.

I used citric acid gel to deal with the surface corrosion, several applications and flap and wire wheel in between. With the corrosion gone I used a 40 grit flap wheel to give the surface a uniform finish, then 60 grit followed by 80, 120, 180 and finally 240 (Starcke Matador papers). Sanding in straight lines

I then began wet sanding from 320 to 400, 600, 800, 1000, 1250, 1500, 2000, 2500, 3000, 5000 and finally 7000. Yeah, it took a LONG time.

Unfortunately minor burrs, blemishes and some very tiny scratches were present. So I went out and purchased http://www.metalpolishingsupplies.c...eel-stainless-steel-11pc-metal-polishing-kit/

I followed the instructions, began with the sisal and finished off with the G loose mop using the corresponding compounds. The finish is certainly better than I achieved with hand sanding/wet sanding. The burrs and marring has gone but the tiny scratches remain.

Obviously a mistake has occurred somewhere, any ideas?

Thanks

 

[the snooper]

If you have tiny scratches you didn't go far enough with the smoother grades, is it worth doing on m/s ? It's gonna start rusting again the minute you use the car

 

[The Z'eer]

If you have tiny scratches you didn't go far enough with the smoother grades, is it worth doing on m/s ? It's gonna start rusting again the minute you use the car

It's probably not worth doing considering the material its made from and the location. However, it's a skill I wanted to learn and so I figured I'd try my hand at it.

I think the problem might have occurred when I tried to take out the scratches left from the 40 grit flap wheel. Perhaps a wire wheel would leave left less scratches but still achieved the desired result? Also should I be varying the sanding technique i.e horizontal lines with one grit then vertical with the next?

 

[the snooper]

Don't think orientation will make a difference no matter which way you will need to remove them with finer grades

 

[hotrodder]

Yep, changing direction (sanding and with mops) is useful/good practice because, amongst other stuff, if the sanding scratches are all going in the same direction it's more difficult to see when it's time to switch to the next finer grit and/or if you changed too soon. Doesn't have to be vertical and horizontal, geometry of the part dictates what's most practical for example with flatish/lightly compound curved parts whether it's blocking paint on a car door or polishing bare metal working at multiple angles is a good idea to help avoid sanding furrows into the thing i.e. vertical, horizontal and then at 45° in two different directions

Don't need to go any where near as far with paper abrasives before switching to a sisal mop and a compund that has lots of 'cut'. Wet sanding with really fine grits is more for painted surfaces before switching to cutting compounds. The starting point with abrasives also doesn't necessarily have to be anywhere near as aggresive 40 grit... starting point only needs to be aggressive enough to remove the deepest scratches in the part in a sensible time unless the surface needs 'leveling' so it doesn't look like like the alps once it's shiney; obviously that's a much bigger deal with flat surfaces than curved. Then you need to remove the sanding scratches with a finer grit and so on

If you search the forum for 'polishing' you'll find some more indepth guides, some good ones have posted by Langy and Scott0031 so can limit results to posts by them if you don't want to go through every post about polishing stuff. There's also a good one here http://thepolishingshop.co.uk/acatalog/Polishing_Guide.html

 

[slim_boy_fat]

Pop over to www.detailingworld.com and do some searching/reading

 

[Rocketman]

You can't get mirror finish in mild steel

Mild steel is full of impurities and the basic composition is not designed for mirror finish.

You can polish it up but a mirror finish would be a stretch.

Put is this way EN8 you can't get a mirror finish on.

 

[langy]

Steel is mirror finished before copper plating & chroming, it looks exactly like chromed before it's chromed if you see what I mean, not sure what the OP is trying to achieve though, the smoother the surface the longer it will stay corrosion free but it will corrode eventually

 

[500e]

Just remember whatever scratches you put in you have to take out, it seems a coarse cut will be quick but later you wish you had listened the old fart

 

[brightspark]

any metal to be chrome plated must be mirror finish before plating so get polishing with finer abrasive and polish

 

[Rocketman]

Twenty years in tool making trade, take it from me mild steel you can't get a true mirror finish on. It's like ali, there is a vast difference between grades on polishing.

Perfect example on steel even if you have a high quality grade with few impurities the amount of sulphur has a huge impact on the polishabilty. While good for machining the sulphur has tendency to drag and leave indents in the steel. Mild steel doesn't have great grain structure and has a reality high level of inclusions just for starters.

 

[The Z'eer]

Yep, changing direction (sanding and with mops) is useful/good practice because, amongst other stuff, if the sanding scratches are all going in the same direction it's more difficult to see when it's time to switch to the next finer grit and/or if you changed too soon. Doesn't have to be vertical and horizontal, geometry of the part dictates what's most practical for example with flatish/lightly compound curved parts whether it's blocking paint on a car door or polishing bare metal working at multiple angles is a good idea to help avoid sanding furrows into the thing i.e. vertical, horizontal and then at 45° in two different directions

Don't need to go any where near as far with paper abrasives before switching to a sisal mop and a compund that has lots of 'cut'. Wet sanding with really fine grits is more for painted surfaces before switching to cutting compounds. The starting point with abrasives also doesn't necessarily have to be anywhere near as aggresive 40 grit... starting point only needs to be aggressive enough to remove the deepest scratches in the part in a sensible time unless the surface needs 'leveling' so it doesn't look like like the alps once it's shiney; obviously that's a much bigger deal with flat surfaces than curved. Then you need to remove the sanding scratches with a finer grit and so on

If you search the forum for 'polishing' you'll find some more indepth guides, some good ones have posted by Langy and Scott0031 so can limit results to posts by them if you don't want to go through every post about polishing stuff. There's also a good one here http://thepolishingshop.co.uk/acatalog/Polishing_Guide.html

A wealth of information there, much appreciated

I used a 40 grit flap wheel to grind off corrosion, markings and deep scratches. Otherwise I would've started higher, 320 grit or so.

With the next piece I'll use alternative strokes when sanding, this way I'll know the scrates are fully removed.

When can the sisal wheel be used? Just after 800 grit is about the right point?

 

[langy]

A wealth of information there, much appreciated

I used a 40 grit flap wheel to grind off corrosion, markings and deep scratches. Otherwise I would've started higher, 320 grit or so.

With the next piece I'll use alternative strokes when sanding, this way I'll know the scrates are fully removed.

When can the sisal wheel be used? Just after 800 grit is about the right point?

I would of started with 180, starting with 40 grit is putting terribly deep scratches in it that you have to get out again, sisal used after 320-400
My polisher friend starts at 400

 

[hareng]

Most weldable quality mild steels you can get at least a 99% mirror finish though 85% been done for over a century. Same with most aluminiums its too easy to over polish and burnish over holes but easy to get 95% mirror finish just got to have the know how.
As above slightest imperfections prior to copper, nickel and gold plating plus blacking will show through like a sore thumb as well.

Pro polishers just take the parts to a grade, whatever imperfections still in that's it. That why you will never get a price for a pukka job, it can be an endless exercise measured in hours rather than minutes.

 

[hotrodder]

A wealth of information there, much appreciated

I used a 40 grit flap wheel to grind off corrosion, markings and deep scratches. Otherwise I would've started higher, 320 grit or so.

With the next piece I'll use alternative strokes when sanding, this way I'll know the scrates are fully removed.

When can the sisal wheel be used? Just after 800 grit is about the right point?

Cheers. As Langy i'd stop at 320 or 400 grit paper. Flatish stuff i sometimes go to 6 or 800 grit instead of a sisal mop and compound with an aggressive cut. That said if whatever is driving the mops doesn't have enough grunt then going sanding with finer grits may be less frustrating... need to able lean hard enough on the mops to generate enough heat to get the compound working, if the machine slows/stalls when this is done then all you end up doing is smearing compound around the workpiece

 

[The Z'eer]

You can't get mirror finish in mild steel

Mild steel is full of impurities and the basic composition is not designed for mirror finish.

You can polish it up but a mirror finish would be a stretch.

Put is this way EN8 you can't get a mirror finish on.

I'm not exactly sure what grade of mild steel the exhaust is, but I'm relatively happy with the achieved finish.

 

[The Z'eer]

any metal to be chrome plated must be mirror finish before plating so get polishing with finer abrasive and polish

The price of chrome plating isn't cheap, and with this piece I won't be going down that route. Thanks for the info though

 

[The Z'eer]

I would of started with 180, starting with 40 grit is putting terribly deep scratches in it that you have to get out again, sisal used after 320-400
My polisher friend starts at 400

You bet the 40 grit scratches were b*tch to get out, I went over the piece many times by hand, dry sanding on 60 grit to remove them. I went on to 600 before I dared to use the sisal.

 

[The Z'eer]

Cheers. As Langy i'd stop at 320 or 400 grit paper. Flatish stuff i sometimes go to 6 or 800 grit instead of a sisal mop and compound with an aggressive cut. That said if whatever is driving the mops doesn't have enough grunt then going sanding with finer grits may be less frustrating... need to able lean hard enough on the mops to generate enough heat to get the compound working, if the machine slows/stalls when this is done then all you end up doing is smearing compound around the workpiece

I'm using a 240 volt De Walt drill for the polishing process, could you comment if this is suitable for this application?

 

[The Z'eer]

Here's how it turned out...



It's far from perfect, but for a first attempt it's reasonable (pitted areas are due to prior corrosion)