Gold plated/gold filled/SGP differences

Recently I started to think about it because I read stuff on different forums.

My basic understanding is that gold filled(GF) has thick layer of gold on top. Gold plated(GP) has a thin layer. And then.. there's SGP. What the hell is SGP? Seiko Gold Plated?

Is SGP really gold plated at all? Some opinions online say it's not even gold but something yellow.

So many Seiko watches with SGP have dulled edges and green spots. Compared to earlier GF/GP watches it seems most SGP ones are worn. Weirder even, SGP watches would often cost more than gold capped(GC) ones, even though gold capped are usually better preserved and seem to have a much thicker layer of gold.

Yet it seems in Seiko's head 18K > SGP > GC > SS.

If SGP isn't really gold plated but something else.. it would be ridiculous and insulting I think.

And what's below the SGP layer anyway - is it brass or stainless steel or? I read that in those years(60-80s), gold couldn't be plated on top of SS.

Thanks for any info in advance. Super curious. I will be posting pics of such watches if I get one. Atm the only GF one I have is a Crown Special I've already posted in the Seiko subforum.(and one Sportsmatic in bad condition, again in the Seiko subforum).

Btw feel free to post information about other brands. It would be interesting to compare Omega's gold-plated/gold-filled designations/etc. and if they have something equivalent to SGP.

Then, how can we explain the dullness/worn-outness on most SGP? Even on the high end watches. On most pics it doesn't really look like gold. I have yet to get mt hands on an SGP watch to interrogate and post photos.

If the plating was much thinner than regular GP/GF how come it was more expensive than GC? Sounds like Seiko cheaped out and expected people to foolishly buy SGP and not GC.

STP - I have never seen one in real life although I've heard of it before. Sounds logical when SS was expensive and most watches were made from other materials. Today I think it doesn't make sense when even watches on Ali are from okay SS. It would really bring Seiko to a new low if they reintroduce this and make watches prone to rusting. And otherwise brass/aluminum/etc. is more expensive than SS, softer too.

If I ever get an STP watch, how could I verify if it's plated in stainless steel? My only idea is those chemicals used to test stainless steel. I don't have them and I think they would leave a mark on the watch case if used. I once wanted to learn to differentiate between SS304 and SS316. Easiest way being just to leave an identical piece in salt water and wait for rust to form.


And out of curiosity, what would you do with a worn-out SGP watch that has otherwise good movement and dial? Would you replate or move to an SS case despite the dial/hands being gold colored and not silver? I am considering getting a watch like this.

You got me really curious about the STP though. Googling some websites in Japanese it seems there's a consensus about the term meaning stainless steel plating. Logically however it doesn't make sense, since it's an alloy. You would need to deposit the pricise amount of chromium, nickel, iron and carbon. Sounds difficult to do in the 60s.

Then, what if you just mixed it? But logically I would expect that the you would get mostly iron plated on the case and not much chromium or carbon at all...

I couldn't find any scientific articles so far or anything. One day if I get a badly beaten STP, I might just cut the caseback and post photos. It sounds incredible for Seiko to have had this process developed back then without any information.

Was that a thing in the 60s though? Nowadays I would argue you could simply 3D print the case from different materials or something.(which is probably similar to the technology you outlined if not the same).

And not only that, but the costs - because it has to be cheap or you will otherwise go full SS.

Apparently Seiko did STP on some Sportsmatic watches and some Crown ones in the 60s. Curiously enough one japanese website I found specifically said they think it was a way to sell a leftover surplus of already produced Crown movements/dials cheaper(after Seiko was already mass producing their next movements) and talked of consumerism implying the inferiority of STP compared to SS. As for the Sportsmatic, it said only a few early ones had STP and then it went full SS.

Gold cap is a different thing. I have appreciation for such watches, which grew slowly tbh. But now I like them. Much better than those flaky GP/GF ones.

My connie is gold filled. While there is some wear after 70 years, it's still intact and looks pretty good. Is it as good as 18k gold, nope no way. But the would put it in a different price category and likely out of my reach.