I used some Dupli-Color High Temp engine paint from O'Reilly Auto Parts. If you don't have one in your area I'm sure others will. The finish looks great (very close to factory). There are 500 and 1200 degree varieties. I used 1200 and it is awesome. Much better than the Rustoleum which I have never been a fan of.
First, don't listen to the helpful hardware man, they have no clue. Just as a confirmation to this - ask them if their base coat of black is red or blue in base for it.
I will have avoided the primer - not needed if the fixture is clean. Also a personal dislike of Rustolum as a brand even if I use it at times less important. Just applies too thick and un-forgiving in spatters for my tastes. High temp. semi-gloss is a special order item either thru them or on-line such as thru Grainger or McMaster Carr for the more prefered Kraylon brand of high temp. paint in gloss or semi-gloss. In Chicago otherwise, some of the theater suppliers might stock it also. Otherwise as a supplier engine paint such as thru an auto parts store will have been better.
End result was flat of what you got. Could work with it, or add some high temperature oil to that finish, than clean it off with multi-surface cleaner. Once that oil is cleaned off the surface, it should result in a semi-gloss finish that doesn't attract dust persay. That's a way of doing it at least - not the best way but a way of doing it.
Otherwise, given it's a flat paint, it's also possible to re-paint without primer with a semi-gloss paint of in my opinion any type - high temp or not in getting back to glossy finish. The high-temp. is on, could apply your third coat to the fixture of another high temp. semi-gloss or in my opinion another coat of just semi-gloss. Three coats is a lot of paint, make your next coat a very light coat.
Hmm, something to be said about a "Vintage" ETC S-4 Leko...
My opinions the above of course - especially about the attempt at using say a Tri-Flo oil coating over the fixture, cleaning that coating off with WD-40 and expecting your flat paint to become semi-gloss, yet a semi-gloss that won't be a magnet for dust and dirt.