OK....one more post to completely wrap this up! Wakeup makes two good points.....first, it does not matter how you achieve surface roughness....either by leaving the sand casting texture (a jumble of mountains and valleys ), or by blasting depressions into the surface. It does not take much roughness to induce turbulence, and like most things in life, a little may be good, but more not necessarily better. Secondly, he points out that matte black is preferable to gloss. Matte black has a higher emissivity (scientific speak for Stefan's constant) than gloss.
For those who just have to spiffy up your jugs, you have my permission to sand and polish the fin EDGES, then matte black spray the jugs, next using a roller, gloss black paint the edges. Theoretically this goes against all that has been said above, but the edges account for an insignificant percentage of the fin's heat transfer, so you do not lose much. In fact, this is exactly how I intend to treat my jugs when I swap out my high compression pistons for lower CR.
Oh....why do I have high CR pistons? Because on a transcontinental trip, I took my Atlas onto the Bonneville salt flats, not knowing the salt was hard and dry only limited times a year, and got brine over everything, which turned white when it dried. Even tho' I was a graduate engineer and should have known better, I failed to wash it off (being 3 days late for graduate classes 2000 miles away, having dallied too Long in San Francisco with a hot mamma, may have had something to do with it). At first, everything was cool (pun intended), but when I got down to lower elevation flatlands, and the air temp. heated up, I overheated the Norton, destroying one piston and bending up a valve. So lest anyone still think color does not matter......