So I haven't had the time to get around it since @sbkdrew shipped it back to me.
I know it's hard to believe, but it's true.

First I needed to get the tiny balls from the tumble treatment out of it cause they are hundreds of them inside it.
The swingarm is not hollow BTW but it is filled with foam.
So getting them out is much more difficult than I thought.
I used pressured water, I was shaking it for an hour, I got a lot of them out but it's still rattling like hell.
At some points my arms started to hurt so I called it a day for now.
It seems I need to repeat the process a lot of times more to get the last of them out of the swinger.

But during this process, a new problem emerged (no shit Maki......or should I say "problems magnet"?)

As I finished with the water, filling the swinger with the hose, after 10 minutes the outer surface of the swingarm started to show black spots where the water drops dried

I used a dish sponge with some detergent and after some soft but painful scrubbing they came off.

But, does that mean that every time I go for a ride and get back home, I have to go through that process all over again each time?

I asked a professional paint shop if they can provide a durable clear coating, that can endure the nature's elements and won't peal off and they told me there isn't such a product that they are aware of.

Remember: no anodizing shops here and I don't wanna powder coat it.
I love the bare aluminium look very much.
I also know for a fact, cause I've tried it in the past, that rattle can clear coat varnish won't last long.

So I did a little research on my own about aluminium clear coating.

1) Aurora Alumetron Clear Polymer Coating for Aluminum

https://www.youtube.com/watch?v=skkvba51DNs

What stroke me that this might be actually a good product is this article

Corrosion Treatment Methods | High Grade Aluminum Alloys

2) Oxidized Aluminum Kit For Motorcycles

This one doesn't look bad either but I have my doubts about the long lasting effects of this product.

Any opinions on those two products form anybody?

Feel free to chime in with any other product or process you know for a fact that does the job but keep in mind that I'm gonna do it most likely at home with what I've got.

For what it is worth, I use shine seal brand aluminum protector for my vintage TRIUMPH motorcycles engine cases. With the higher temperature than you will experience on the swingarm, the protection lasts for 6 months.

Good Luck

Team FASTLIKEJUDY

Pardon my ignorance, but how did the blast media get inside the swingarm? Couldn't this be prevented?

I'm not sure buddy........I guess it couldn't

Any insights on the Alumetron, anybody? A friend you could ask about it and it's performance?

I think I'll follow that path but can't be 100% sure of the outcome in the long run.

The ceramic media particulates are small enough to get through the smaller openings. The RC swing arm is a prime example of an item that has its drawbacks when tumble media polishing. One can plug all the threaded holes with screws but the others are next to impossible to seal off. Anything inserted in a hole has to be secure enough that it does not fall out while being tumbled. Any foreign item tumbling in the media can ruin what it being polished and plus you have to find the loose object so it doesn't ruin the next item(s) being done.
So, if you are pretty familiar with every orifice on the SP2 swing arm, it is very challenging.
The guys at the shop that do the polishing put in a fair amount of effort to remove as much of the media as possible as do i myself, s I know how much work it takes to try and shake them out. After doing quite a few of these I've just accepted the fact that some media may just as well take up permanent residence inside as they are not really going to hurt anything in the long run.
I'll have to check with the shop that does this process and see if they have a station set-up with larger size media maybe but I'm pretty sure they are a certain size for a reason.
Smaller media size particulate for a finer finish.
But I understand doing things to the greatest detail also and having some media floating around inside can be bothersome for some perfectionists...
Not a rant, just telling yall how I approach/accept it.

Went through a different kind of hassle awhile back when I tried using a different powder shop to do the black powder that I do them in. Went there just merely because they were down the block from my work and it was more convenient to go there and they had the correct matching color I needed.
Up until then I was familiar with the other shop, I had done business with a number of years, and their process to prep parts/surfaces for powder.
This other shop had a phosphate chemical rinse they do on all parts they do before entering the spray booth or something with just became a huge problem when doing the swing arms because the cleaning agent would get trapped inside the swing arms and get absorbed by the expanding foam that is inside them. This would make them weep/leak out as they cured in the oven and then ruin the surface finish in the areas the liquid would leak out. I fought them tooth and nail on the whole thing and they had to do them three times. The third time around i got charged extra for cleaning labor. Never went back there since because of the hassle and all the trips I had to make there.
All this is just to illustrate the struggles/endeavors that I've gone through to make sure these work when re-finishing them.

I totally understand that buddy, that's why I never complained to you about the rattling noise the swinger was doing when I took it out of it's packaging.
I have done a lot of sandblasting myself to know that the sanding/tumble/whatever agent is used, eventually is gonna be literally everywhere in the part if that has enclosed spaces on it.
I have thought about leaving it rattling a bit as I have removed the majority of those tiny balls out of it but since it'll take sometime to resolve the staining issue, it wouldn't hurt me to shake it for a couple of hours every day to get some more of them out as well.
Who knows.....maybe I'll manage to take them all out eventually.

I figured that much myself cause I know from previous experiments I've done that aluminium is a bitch to paint in such a way it can withstand small road gravel chips and dust hitting it while riding for more than a few months without the coating start pealing off and shows marks and scuffs on it.

Before I ship it to you, I tried to sand the scuffs it had with some water sand paper and I found out that Honda has it painted from the factory. A very thin and durable silver coat that is definitely NOT powder coating. Looks more like autoshop professional painting but it has been treated in a way that it won't come off even with sand paper.
Most likely oven baked treatment.

I went to my storage and found an NC29 CBR400RR Gull Arm and an MC21 NSR250 Gull Arm swingarms I have and sanded them with sandpaper on a random spot and it turns out they are NOT spray painted like the SP2 swingarm was.

That means that Honda didn't use to spray paint their swingarms on models prior of 2000.
And those swingarms don't stain with water no matter what kind of water is that or what temp it is heated on.

I concluded that they must have used some kind of chemical treatment on the aluminium surface so that it remain neutral and not react to nature's elements it might come across during riding.

That's why I tend to believe that since I don't have it powder coated, the next best thing I should be looking on, is some kind of chemical agent that will neutralize the swingarm's surface.

And all the above wouldn't have happened if it weren't of those tiny balls stuck in the swingarm......

I remember I think mentioning auto-trim clear paint..But that makes it a semi-gloss/flat finish and it is not ideally what most would desire on polished effect finish.
That's what I used on mine a long time ago before I went to black powder.

And geez Makis.....I hope you have a Ti axle in the works or have one already.....yikes.....

I have a brand new OEM one......Ti was too much for my budget.

The one in the picture is the one the swinger came with, and I am just using it to do the measurements.