Hi there !
I forgot to answer your first message. Sorry for that.
First of all, some info to finish this thread :
I eventualy chose to powdercoat the jackplate. My local powdercoater really struggled with this, he had to do re-do it 3 times. There was indeed some chemical reaction at the pitting corrosion pitts as described above. However each time result improved. End result was not perfect, but I'm happy with the 95%
It didn't cost much either, despite the 3x rework. I still had a lot of work with the assembly. All threads needed to be rethreaded, quite a search to find the correct threadtools. All the holes for the pins needed to be reworked to original diameter with a small grinder tool (Dremel). I was more than happy with the end-result. It looks super on my transom. Also the SS transomplate was polished. Long story, took me about 10 full days, combination of little carefull weldings, grindings and polishing... I had already some polishing experience as you will read below, so I knew what I had to do
Now, to answer your question about my polished lower unit :
Yes you have guessed it -> this took a lot of elbow grease and time in order to get this endresult. About 3 months of work, every weekend, some evenings, I lost track of how many hours exactly. I balanced between doing this job and keeping the misses happy, that is very important. I started out with a painted lower unit which started to show some minor blisters, the endresult you saw in the picture above.
I started removing the paint with a steel brush on a grinder. Quite a rough method but not the correct one, I should have used a chemical paint remover, it would have saved me time and a lot of paintdust. Nevertheless for almost all steps you need to wear a P3 mask, earplugs and goggles. What I discovered underneath the paint was quite shocking, there was pitting corrosion underneath the paint. Minor blisters on the outside paintlayer had covered a bigger problem yet solvable so it seemed. As I chose to go for a mirror polished look, I realized I was in for some serious polishing job. The pitting corrosion was only surface deep, I needed to take just a tiny top layer to get it right, after all, I needed to keep mechanical strenght of my lower unit. I used my grinder with special 3M abbrasives : I used 5 grades going from rough to very fine. I used the 3M roloc system which allowed quick change from rougher to finer disc pads. Those disc pads mount on a plastic flexible holder that mounts on the grinder. This allows you to go over the curved surfaces fluently. I also used a smaller diameter 3M roloc system and disc pads for the parts that I couldn't reach with the bigger diameter. you can see it in this link I was amazed how good these things work and I allready took out all pitting after this 5step stage. But make no mistake, you cannot skip any step, you have to work from rough to fine in all 5 steps or it will show. I needed to mark my discpads on the back 1-5 to avoid mistakes, they have different colours but you keep mixing them and they get dirty and look alike after a while.
After this a got to the sanding. First dry sanding P120-P400-P500-P800. I bought special sanding sheets with soft mousse on the back as they use on cars. After the dry sanding I did wet sanding P1000-P1500-P2000. Use enough water. After the wet sanding it looks allready quite shiny.
Time for the next step : polishing. First I cleaned up the whole lower unit with a standard alu cleaner and microfiber towel. This is already mirror-like result. Now, It took me quite a search to find the right polishing kit. I ended up buying a small professional kit from a famous car restauration company. The difference with the material you get in the local do-it-yourself shop is incredible. So get a good polishing kit suited for alu polishing with the right instructions. My polishing kit mounts on a drill. If you donít have a professional drill you will mess up your bearings as you apply lateral force. As I already sanded upto P2000 I found out that I could do with only 1 polishing step. The difference between the cleaned <-> polished stage is enormous. The polishing itself is relative fast, so this is a very rewarding step. But be carefull, when you reach the sides your polishing wheel will catch the side and might hit something and damage an area youíve just polished. If you polish in the correct direction there is off course no risk for this, but in some cases you cannot reach every part of the lower unit and you have no choice.
After the polishing there can be some minor black residu from your polishing bar on the surface. It is easily removed with a microfiber. End result is a true mirror !
Disadvantage is that you see any minor differences in surface height, as in a bit deformed mirror image. To get it right you have to start over and sand it to perfection, this is again a lot of work and in my experience a feeling during sandingÖ
Now if you put this kind of work into it, you want to preserve this result :
First of all, your polished unit is unprotected, so the least you must do is get your anodes right. As I only go boating in fresh waters, this has to be magnesium anodes. I had the wrong ones which caused the pitting in the first place.
I noticed that whatever you do, you cannot preserve the mirror like look without doing some maintenance to it. In my case itís sufficient to repolish once a year, which takes me 0,5 day. For more info on preserving this look you can take a look at this thread -> http://www.boatmad.com/forum/f5/how-...nit-24238.html
So this is quite some work and itís for looks only. So that means, when itís out of the water. In the water during boating it might even be just a bit slower. I think it is true but hardly noticeable.
Iím convinced this polishing job can be done faster than I did, but it will take time. But if you are as crazy and determined as I am you will get good endresult. It was a nice winterproject and I learned how to polish.
Good luck ! T.