Soda Blasting

[FourPoints]

So I picked up everything to do small (as in a few square inches at a time) soda blasting jobs from harbor freight last week. I am trying to clean my lower unit so I can repair some corrosion pitting where the P.O. let the zinc rot away, and I am finding the soda won't cut through some black material that is over much of the lower unit, particularly in the corrosion pits that I really want to clean. It removes the paint fine and leaves a clean aluminum surface, except where this black stuff is. It seems impervious to soda.

I was running it at 100psi since that is all the air dryer is rated for, do you think blasting at 150psi (Max on the compressor) without the air dryer in place would help, or do I need something more aggressive than soda?

Also any idea what the impervious black stuff may be?

 

[splax]

I had my bottom and keel soda blasted this Spring. Like you, I didn't find the soda blasting tough enough. Epoxy and rust still covered my keel. Good Luck.

 

[FourPoints]

I ended up giving up and taking it to a local machine shop that does sandblasting.

He still wasn't able to get all the black stuff off without risking holes through the lower unit. One hole actually did punch through, but luckily for me it was about 1/2" behind the gearcase, and the inside is just open to the water / exhaust anyway, so there's nothing harmed.


I filled all the pits yesterday with Petit EZ-Fair (according to the Petit rep at the defender sale it's very similar to Marine-Tex, it's 100% water proof and designed to adhere to aluminum, but comes in a caulk gun tube so you can easily control your batch size, I hope it really is as good!), I still need to sand it down smooth and see what needs another coat.

 

[jibes138]

Aluminum oxide is pretty tuff stuff, they use it for the abrasive to make grinding wheels and sand paper. Hard anodizing aluminum creates a very thick (several mils.) extremely hard layer for wear resistance. It has a black color to it, so you are probably looking at aluminum oxide in the pits. It is not necessarily a surface contaminant but rather a part of the actual metal, so removing requires taking stock off the actual part. That's a lot more difficult than removing a layer of dirt or contamination. As you can see from the pit depth the black could be very thick also making removal difficult. You might want to coat with epoxy a few coats and then paint with a 2 part paint of some type to further protect it since the anodize is all gone with the sandblasting.

 

[FourPoints]

Once done I'm going to coat all exposed aluminum with a Zinc Chromate outdrive primer, as many coats as the instructions allow for, then finish with at least 3 if not 4 coats of outdrive paint, and then an additional 2 coats of Petit zinc outdrive antifouling paint.

Between the epoxy, zinc chromate, and the waterproof outdrive paint, I think I should be well protected.

 

[FourPoints]

I finished the lower unit repairs

Full details are here:
http://sailingit.com/blog/boat-projects/lower-unit-repair

but if you just want to see the final product...

Before:


After:

 

[jibes138]

Looks good Brian. Let us know how it works once you test drive it.

 

[Ken Cross]

Wow. A lot of work, but it turned out fantastic.