Wet sanding with an air ROS sander?

[julianf]

I'm trying to make some 1000 series aluminium boxes scratch free, but NOT have a polished finish.

I have a Mirka ROS 650CV. Or at least i think thats what it is. Like the one at the end of this mail, apart from mine is so much less shiny that im almost doubting its the same thing!

Anyway, so far ive tried going up to 800grit with a mesh paper (arbanet) and no backing pad (ie the mesh direct onto the replacement velcro i fitted ages back)

The finish is not so bad. There are some areas with significant swirls, where i wonder if i took off the sander at an angle, or pressed to hard, and the velcro hooks made contact or somthing? I dont know.

Ive been on youtube and people talk about wet sanding with ROS units, but im unsure if mine is good for that, and all the stuff i ever find is talking about different units anyway.

And by "wet", i assume i need some sort of "product" rather than just slinging on water? You can tell, i dont really understand this stuff....

Backing pads? Do i need them?

Im trying to work on folded alloy boxes, so there are corners to catch on, so foam pads are out, as they will just tear up the moment i touch a corner with them.


Would i be better sand blasting them with a very fine media, once i get set up for that? All im wanting to do is create a uniform finish - i dont really care what finish that actually is, so long as its uniform.

Basically, they sometimes get scratches in them, and i normally put those ones aside for paint, but, right now, i want some in silver.

I figure on going over them with some sort of car wax stuff at the end of it all, to stop instant marks from finger prints (ive confirmed this works fine) but its just getting the uniform finish to start with that im having issues with. Again, id rather not have a gloss finish. The P800 is almost there, if i could just get it more even, which is why im wondering about wet sanding, but i may be well off course.

Thank you.

 

[wristpin]

May be of no relevance but yonks ago there was an episode of Wheeler Dealers featuring a guy who specialised in taking dents out of De Lorian stainless bodywork and then re-texturing the surface to a uniform surface. I think that he used a horse hair mop. Maybe worth a look if you can find the episode.

 

[mtt.tr]

Sounds like you want a burnisher

 

[Dcal]

Why wet?
Also why so fine?

I would use Mirka Aberanet mesh discs and an intermediate foam pad.
Go up (down?) through the grades probably starting with 120 and keep taking the scratches out with finer grades until you get the finish you want.
I would have thought 400 would be fine enough.
If wet works better just use the Aberanet sheets with water.
If you want an even finer finish, change to Mirka Abralon discs which go into the 1000s grades but the finer you go the more polished the finish.

To properly remove a scratch you need to remove all the surrounding material until it is the same level as the bottom of the deepest scratch which could be a lot of work.
Also dry sanding gives a finer finish so 800 grade wet is about 400 grade dry (roughly)
A very fine dry blast or vapour blast might be a quicker option.

 

[Edward Teach]

Scotchbrite pads/discs possibly,
Though they may 'catch' like a foam, intermediate backing would, on the sharp corners.

 

[julianf]

Some photos are below. This is with 800grit

Mostly its ok, but you can certainly see the bit in the corner that i mentioned prior. There are other areas where things have broken down a bit also, like on the 45 degree parts.

I understand about removing the larger scratches - thats not an issue, they are not deep and can be cut out fine, but its restoring an even finish thereafter thats the issue.

Its only for cosmetics - these are not surfaces that are to be painted etc. so im tying to get the most uniform look that i can on the bare alloy.

Thank you.

 

[colnerov]

Hi, Two techniques I've learnt, if you don't know already, Is to keep the air pressure up and the physical pressure down. Higher air pressure keeps the speed up and leaning on it too hard slows it down, so you then lose the random orbit action and it just tends to spin. The other is to sand off the edge or corner not onto it, so is less likely to dig in.

In short you just put the egg in your mouth and suck.

Colin

 

[Kram]

i dont really care what finish that actually is, so long as its uniform.

As you mention aluminium, I was suprised how uniform my parts were after a 1-2 minute, 20% NaOH caustic dip. I guess it takes me about 5 minutes per part with a degrease, dip, rinse and dry, probably works out better than blasting if the existing finish is not too bad. It hid any fine file marks, machining lines and scraches but not coarse file marks.

 

[julianf]

As you mention aluminium, I was suprised how uniform my parts were after a 1-2 minute, 20% NaOH caustic dip. I guess it takes me about 5 minutes per part with a degrease, dip, rinse and dry, probably works out better than blasting if the existing finish is not too bad. It hid any fine file marks, machining lines and scraches but not coarse file marks.

I was considering this, thank you.

When you say "20%", how do you work that out? Basically, i have a 25kg sack of NaOH, and a barrel of water - how do i go about calculating the 20%?

I need to have another look at your anodising thread again too - im wondering how hard it would be to add a natural (no dye) anodised finish to these too.

 

[Ruffian]

320grit and high speed.
Gives a nice uniform finish.

Until you get it uniform and the deeper scratches out no point fighting with 800grit.

 

[Kram]

Edit: Im an idiot, 17% is what Im using.

I do my sciency percentages by weight, 200g per litre works for me. Wikipedia tells me its also about 20% of a saturated solution, 1kg/L.

Anodising, I dont know if it was luck or just easy to get right, but it went very well, however it took a couple of hours and would suit doing a batch of many parts together.

 

[Ruffian]

I was considering this, thank you.

When you say "20%", how do you work that out? Basically, i have a 25kg sack of NaOH, and a barrel of water - how do i go about calculating the 20%?

I need to have another look at your anodising thread again too - im wondering how hard it would be to add a natural (no dye) anodised finish to these too.

1litre of water is a 1kg.
Therefore 4kg of water and 1kg of Noah would be right weight wise.

 

[julianf]

320grit and high speed.
Gives a nice uniform finish.

Until you get it uniform and the deeper scratches out no point fighting with 800grit.

Yeah, sure, i think i created a red-herring by ever mentioning deeper scratches. I can, and have, dealt with them fine -

Its the final finish that i have issues with. Im not talking about making it flat, or smooth, im talking about the real detail about how it reflects etc. ie cosmetic rather than mechanical. (i mean obviously they are the same, but i hope you see what i mean?)

 

[Ruffian]

Well in that case with an orbital it wl be a consistent finish but not uniform, hence the random.

You would want to put a scotch disc on a burnisher as others have said to get all the lines one direction.

 

[Hitch]

Have you/can you turn the rotary part of the action off? So just orbital...