Darker Rougher spots on engine case. What are my options?

I have soda blasted the front side of the bottom end of the engine and the lower case half seems darker, and rougher.



The left side is cleaning up pretty good, the right side looks pretty sad. I used some emery compound on a small spot to see if it would help, and it put too much of a shine on it. Ide like to have that clean casting look, or am i just going to have to polish the engine cases? lol

 

It is painted and from operating heat the stuff is baked on ... The dark area is where paint is actually removed.. Continue to blast or chemically strip it - will take 3 or 4 strippings - then blast ... It's a slippery slope once started

IMHO

 

I haven't found any left over paint anywhere that would lead me to believe anything but the fins and valve cover have ever been painted.

 

I believe the entire engine, case and all, have a silver paint on them (midnights are otherwise painted) I believe you're seeing partial paint removal in your "spots" - the alu (when paint is removed) is actually darker until polished

 

Most likely what you are dealing with is the clear coat applied to the natural castings. You can strip it off with airplane stripper. Then do your soda blasting to get clean natural castings.

You can give those castings a nice textured finish by using crushed walnut shell in your blaster.

Ghost

 

They look the same as the top half of my 750, it's a silver paint. The bottom case on my engine is natural finish only, no clear coat. Soda will take forever and IMHO will not give you the best finish.

Add 50% by volume of glass beads. It is aggressive enough to take the oxide off and the soda will give a polishing effect. Works a treat.

Go here:

http://www.xjbikes.com/Forums/viewtopic ... rt=15.html

and look at the dark spots on the carb body in the middle. That was soda blasted for about ten minutes and wouldn't brighten up. The glass beads will make a difference that will amaze you.

 

Rudie; the cases are finished as described by dmcoach and iandmac; the upper engine cases were painted, the lower half left raw. It's some seriously tough silver paint, but it's paint.

Here're a couple of closeups of one of my 550s that illustrate it very clearly:

 

Thanks to everyone who chimed in, had no idea about the paint. Fitz's pictures are exactly what I'm looking at, except mines a little uglier.

Thanks mac, for the media recipe. I will definitely be giving that a try.

My question now is, should I actually try and strip what's left of the paint, or leave it?

For one, I'm sure it would be a pain in the ass, but what about protecting it once its stripped? The glass/soda blasting finished product does look quite sexy, but am I going to have to clearcoat it, or would maybe buffing in some good polish hold up?