A question that has nagged at me for many years. How do frame builders chrome just the lugs of a frame? Are they chromed before assembly?......I apologize if this is the wrong forum. If I learn the answer to this question I will die a happier man.

Sometimes they just flat out chrome the whole frame and paint over it. Though, I imagine there's a masking tape of some sort that can survive the acid/nickel/chrome baths it would go through. I'm no expert, but, I do know a little about the chrome plating process.
-Gene-

They chrome the whole frame after its built and pay "special attention" (extra layers of chrome) to the the lugs, then paint. I may be wrong, but then again.....

It's a special present from your chosen divine deity.....

First, the lugs are surface finished before the frame is brazed. When its complete, the areas to show as chrome are polished smooth. For plating, they can dip the entire frame or just the head and stay areas, but only the polished parts will look chrome, smooth and shiny.

Chrome plating is a series of dips in acid baths that desolve the metals to be plated: copper to fill, nickel for the silver, chrome as a hard shiny surface. The acid breaks down the plating metal into the solution, electric current is run through the object to be plated and the bath, and the molecules of the plating metal are attracted to the object. Triple-chrome plate really means copper, nickel, chrome. Prior to the 40s or so many things were nickel plated, which is a softer, warmer look than chrome.

Back in my plating days, we'd put fingernail polish on the parts that we didn't want plated and when we were done, we'd use nail polish remover (acetone) to take the polish off.
But most bike makers just dip the front part of the bike and paint over the rest.