I am considering building up a cx bike with parts from my roadbike. I found two affordable frames and both have a brushed aluminum finish. It sure looks nice but I'm worried about the lifespan of these frames. Would I need to treat these frames with something like a clearcoat? Does the bare aluminum last as long as a painted frame would? Thanks!

Its still aliminum painted or not. makes no difference, i like a brushed frame personally.

Yeah, it makes a difference.And they are clearcoated to keep them looking good.Corrosion could also be an issue in some conditions.

I have a brushed aluminum frame (a LeMond Big Sky). Lots of people think its Titanium! Style points for less money. Ha.

On the other hand, it scratches just like a painted frame, although it may not be as apparent. And it seems kind of soft (i.e., scratches easily).

I like it though, all things considered.

Al oxidises but unlike steel, the oxide forms a protective barrier preventing further reaction. Anodising will thicken the barrier and you can stain rather than paint the oxide layer.
Leaving bare metal is cheaper, lighter and after hard use, it keeps its looks.
You may want to apply some car wax to keep mud from sticking but its quite OK to use the unpainted metal.

You will want to wax it to prevent it from turning white with advanced corrosion if you ride winters. Polished is a bit more work but I think the results are superior. Clearcoat or silver paint is a much lower maintenance option. There are powdercoats and durable wet paints that can duplicate any look from TI to chrome now for decent cost.

Can you recommend a clearcoat brand that's good for bike parts? I buffed out a crankset and would like to reapply some clearcoat, since it loses its shine pretty quickly right now.

I would look into an automotive paint shop such as NAPA or the like. At the car shop I work part time at we use them and the prices seem decent. Not all of them do paint though, they will have a sign on the outside if they do. Don't get the spray can krylon stuff. It is way cheaper but believe me it ain't worth it.

I, personally would not clear coat an AL frame. AL corrodes, but this type of corrosion is not destructive (unlike rust is on steel). Just hit it with AL polish now and then when it looks a little white. I have an 8 yr old polished MTB frame that is going strong. I use a little Mothers AL polish and it look s like a mirror.

The last thing I would want is clear coat chipping off.

The automatic "skin" corrosion is one thing. If left to its own it will be alright. But given moisture it will deteriorate to the white stuff and eventually into flakes and badness. We deal a lot in that stuff in the plane repairs at the hangar. In the right conditions(wet hangar usually) they can get to that stage in a matter of months. Add to it road salt, which planes never see, and you can have an ugly situation. That is not to say the frame will neccesarily go that route, but it would very easily do so.
When you polish the AL the polish has a twofold effect. The polish will remove the dull skin corrosion and buff to a smother surface, creating the basic shine, then if it is a good polish it will have waxes in it as well as a "passivation" agent. This agent would react with the surface to prevent the skin layer formation until it wears off. Not all have the agent, but almost all have waxes.

I vehemently disagree with the "it keeps its looks" park. I would agree if I had not had the pleasure of buffing out 3 unfinished aluminum GT frames a few years ago. They looked like crap after a few years of use, so since I was a detailer, one of the guys I worked with brought me his, and then two of his friends brought me theirs to buff out. Definately DID NOT keep their looks!

THats why you don't see bare polished planes anymore. We had one at mechanic school and it used to be a class project to buff it. it would last 2 months before it looked like it was never done. Paint with a chrome-like paint and clearcoat it and viola, it is good for paint lifespan.

Tried a couple coats of johnsons hard floor wax?