Getting between the fins is not easy both to make certain that the area is clean and to key the surfaces.
When I did this bike seven years ago, I used a fine emery cloth (instead of wet & dry paper) wrapped around something like an ice lolly stick.
The main engine parts of this bike were painted seven years ago and not re-touched since then. I've only done the valve covers and air-box again because I had them off and had to paint the recon gearbox to match the engine finish.
Purists will never like the idea of painting parts that were not originally painted. However, this bike isn't a Concourse entrant. There's probably only a little less effort required to paint the parts than to thoroughly clean them. The real difference is in how well they retain the finish and how little effort it takes to stop the effects of use undoing the work.
I'm fairly certain that BMW would have painted their older Boxer engines (had it occurred to them) long before they started doing so with some of their early K-Series and as they have been doing so ever since.
Apart from careful preparation, the choice of paint is important, it obviously needs to be heat resistant enough to cope with the highest temperatures (near the exhaust ports).
There are a number of off-the-shelf engine and heat resistant paints available in aerosol cans. Over the many years I've been satisfying my urge to tart-up cars and bikes I've tried a few. All benefit from some "curing" after application. This may be leaving it for 72 hours or baking it on somehow.
Enamel paints have very good heat-resistant properties and Finnegan's Hammerite has become my preferred option. A few years ago I tried Simonez "Tough" on the engine cases of a K75 that I owned. The satin black looked nice:
(it's still wet in the above and dry below)
It takes a long time to properly cure and is easily marked until it has had some heat cycles through it.
It was still looking good when I sold it a few months ago:
Hammerite, on the other-hand, needs little time to cure and is much more robust from the off.
Satin black on my 1150GSA:
This image was taken nearly 12 months after the "tart-up":
What I would advise is that when it's all painted, don't be tempted to jump-on and go for a long ride. Warm the engine in short stages to finally cure the paint. Too much heat too soon may cause it to shrink and crack. The paint may be dry to the touch but it might not have fully dried.
