We start with the standard piece, in this case it's the coolant neck of a Gen3 3S-GTE engine.
The first step is the most time consuming and important phase. The amount of work and detail you're prepared to bring into this phase determines how well it'll look in the end.
To get the part to a smooth, mirror-shine finish we need to smoothen the surface of the part. Since this aluminum part is cast, the cast structure needs to be leveled. Aluminum is very soft and this can be achieved by hand-sanding the structure down with grit 80 sanding paper:
I'm using a cork sanding block, which isn't too large and gives in a little. A hard, plastic sanding block wouldn't work as well because it's not flexible, making it much harder to sand curved surfaces.
After the first stroke, this problem area becomes visible:
The top is not completely round but has a weird shape. You could leave it like this, but as I said, this stage determines how well the finished piece will look. So I want this removed to create a nice round shape instead of what it is now. I use a big metal file to level the high areas. This is the result:
I also filed down the casting seams at the top:
The rest of the shapes were okay, these just required sanding. Do as much as you can with the sanding block, because you can easily apply pressure on it, making the work much easier. Also, when not using a tool like a block, you're bound to create waves in the soft material, which will show once it's completed. Compare it to body filler: you need the sanding block to create a tight, planar surface. If you'd sand the filler by hand, you'd create a warped surface.
After sanding more:
Here I filed away the ongoing casting seam with the same file:
The file leaves pretty heavy scratches. 80 Grit sandpaper deals with it though:
A bit further down, there's a small '2' cast into the piece.
I used a miniature file to get rid of it:
I used the large metal file again to level this:
TIME NEEDED TO COMPLETE 80GRIT SANDING: 6-7 HOURS
The next stages are considerably shorter, and easier. 120 Grit sandpaper is up!
In this picture, the top part has been sanded down with 120 grit, the rest is still 80. The idea is, that every scratch you made with the 80 grit sanding paper must be removed by the 120 grit. If you don't, you'll see it throughout the remaining stages!
Here, the piece is complete done.
TIME NEEDED TO COMPLETE 120GRIT SANDING: 1.5 HOURS.
The next stage is 220 grit sanding. Again, you should sand away all the sanding marks left by the 120 grit stage. All done:
TIME NEEDED TO COMPLETE 220GRIT SANDING: 1 HOUR.
The next phase it 400. This is where it gets tricky, because this is one of the highest grits you can still dry-sand. Wipe off the paper often, because it tends to fill up with aluminum dust very quickly.
As you can see, the piece gets shinier by the hour!
TIME NEEDED TO COMPLETE 400GRIT SANDING: 1 HOUR.
Now we move to wet sanding. Get a bucket of warm water and throw in some pieces of 1200 grit sanding paper. You might also get an old towel, fold it four times, and lay it on your lap. I found this greatly helps in not getting my lap soaking wet.
Also, note that at this point, sanding can be done by hand for most parts. You're not altering the shape of the piece any more, you're just refining the texture of the aluminum. Some parts may still be easier to do with a block or rubber, but hand-sanding will get you into all the tight areas.
TIME NEEDED TO COMPLETE 1200GRIT WET-SANDING: 45 MINUTES.
The last sanding step is 2500 grit. Again, this can be done by hand to save time. Finished piece:
TIME NEEDED TO COMPLETE 2500GRIT WET-SANDING: 45 MINUTES.
Now it's time for the topping on the ice! Get yourself a hard cotton/cloth polishing disc with polishing paste (usually a hard bar), and mount it on a drill.
Use this to polish the piece to the mirror-shine you always dreamed of!
TIME NEEDED TO COMPLETE MACHINE POLISHING: 15 MINUTES.
Now get polishing people!!
Met onzettend veel dank aan Edwin, Ilpoto, voor deze zeer duidelijke instructie over het polijsten. RESPECT EDWIN!!!