Picture attached:

I plan on tackling minor subframe rusting soon on my Civic. Has anyone done this? Is this a good idea? Bad idea? Something I'm not aware of? It seems simple to me, I just want to lightly sand away minor surface rust then do four-ish coats of chassis black. I really want to do a metal primer first but I can't find any that I can brush-on since I don't want to spray all over the place.

The surface rust is on mostly the back of the subframe where I assume splashes and road debris smack into it (shown in the picture). It also occurs on other areas like the flat area of the subframe on the left and right sides by the control arms as well as the left and right angled tow hooks. Apparently the previous owner must have drove over something because it's scuffed and rusted.

I also noticed the transmission case is showing something that looks like rust. but I always thought the case was made of aluminum like the engine block? It even has the same white marks that look like aluminum oxide that the engine does. Is this going to rust through or is this just maybe rail debris or some other metal road debris that embedded into the surface and is starting to rust?

 

The case is aluminum, but the bolts are steel. The big circle you have on the transmission case is below the drain bolt (square 3/8 inch hole). The other rust stain has a corroded bolt above it too. My guess is these are rust stains. I also doubt it will corrode through the case.

For the subframe, maybe clean it up with some light scuffing, then put on a coat of rust converter. Then paint. Maybe others can offer some advice on this.

 

The issue with the bolts is from dissimilar metals. I would suggest replacing the rusted bolts. Maybe the plating wore off of them. They are suppose to choose plating to prevent this.

 

X2. Wire brush, compressed air, then rust converter. Purple Power "De-Ruster" is available in the stores around me. The phosphoric acid turns iron oxide into black ferric phosphate. Leave it dry for a day or so then wash off the film with soap and water. Top it with clean metal primer then paint, or just Rustoleum "Stops Rust." Stops Rust is a clear, semi-gloss coating that is sort of oily until it drys.

 

How is the fuel/brake line bundle running down the center of the vehicle holding up?