Ian, apart from Triumph 3HW's and as far as I can think, the factory finish for exhausts were dull chrome. (not an easy option these days). A silver/aluminium VHT paint from a rattle can will do the job and no primer needed. But of course it will usually burn off at the barrel end. However I have blasted bright chrome parts with Aluminium Oxide grit for a dull finish. Like my 16H here. I would prefer to buy a stainless system to blast, but no one makes one for a 16H
Ron
email (option): ronpier@talk21.com
I had mine done with the hot metal spray 'Schoop'- system: aluminium/zinc is wire-fed and sprayed on a grit-blasted surface. VERY resistant to heat and rust, it seems that during the war this was also applied to some 16H. And after the war on the Dutch Army 1967 Triumph 3TA DR bikes 
email (option): viaconsu [at] planet'dot'nl
Good quality, high temperature silver spray paint for a barbecue did the trick for me. I bought unplated exhaust and silencers, keyed them up and sprayed them two years ago and they still look fine. Cheap and easy.
Show me the pictures...I find that hard to believe. How many miles have you covered in that 2 years?...and how many of those were in the rain...?
I tried everything before changing to stainless systems for my WD bikes and never found a paint that would stay on for more than a few weeks use in variable conditions, particularly so on salty, wet winter roads...Ian
To be honest, I actually painted the exhaust with black barbecue paint first, didn't like it and painted over that with the silver, so it is actually black over silver, but it's just standard off the shelf stuff and I've done over 1800 miles in sun, rain and snow. Sorry if it's unbelievable Ian, but it's true. I don't think it will out last your stainless system, but so far it seems to be OK. I don't have an account for up loading photos, or is it downloading? What's the difference? Anyway, if you really need proof I can send something to your e-mail? 
No pictures needed really Bill... ...Can't say I ever managed it though... ....Ian
['consider that mud, dings, rust and cannibalised parts are more faithful to the past. IMHO.']
At the risk of starting off the 'endless debate' again, show me one that came from the factory like that...
Oxford Dictionary...'Restoration'...'The process of returning **** to its original condition or state'.
The above definition isn't possible for various reasons...It is only possible, in the strictest sense, to create a 'representation' of the 'original state' via the process of restoration.
However, I think you must accept 'mud, dings, rust and cannibalised parts' aren't 'original' features either and don't represent 'the original state' in any way...
Most of my restorations were well beyond that point to start off with anyway and HAD to be restored but I don't claim any of them are original or even to original specification in every way......
It is a misuse of the definition and the language to call an old, worn bike 'original'...it plainly isn't as it was when it left the factory...
'Unrestored' would be the correct term....but neither a restored bike or a worn unrestored one are 'in the original state'..
Just walk into any motorcycle showroom or factory to prove that is the case...
Anyone who thinks their old battered bike is 'original' is deluding themselves just as much as someone who thinks their restored bike is, in my opinion..
I've found bikes go just as well in either condition so ultimately it makes little difference to me.. I just find the hypocrisy of it annoying... Ian
email (option): ian@wright52.plus.com
'Ex Factory' (for want of a better term.) bikes are still in the minority in the military scene I would say..though these standards are being applied more commonly as the bikes become more desirable to a greater number of people and accrue similar, or more, value than equivalent civvy models.
I restored my own M20 with all the dull chrome/cadmium and sprayed finish a long time ago and frankly, most of my friends thought I was mad to 'waste' all that money on 'an old army bike' at the time...
As regards accumulated wear and damage...?...I've done well over 100,000 miles on my M20 over nearly 30 years and including the initial rebuild its been through the full cycle of rebuild/patina/worn out two more times...In fact its currently undergoing the third rebuild at the moment...
It still has a very good set of original major parts but I don't claim it to be 'original'...Also, for the sake of durability I have happily altered the finishes and in some cases materials to make the bike more durable and extend the periods between rebuilds...and I make no apology for that as it is me that has to do them!...However, it still pretty much 'looks the part'...
As these bikes get towards 100 years old I think it's best to accept that they have to go through this 'cycle' if they are seriously used (as mine has been)...It doesn't matter how 'original' your bike is, if you use it, it will be in a constant state of change and periodically you will HAVE to rebuild it.....
'Patina' and 'original finish' are the preserve of people who won't use the bike enough to wear it out to the point of refurbishment..a 'luxury' I have never afforded myself as I have used old bikes as my main transport for the last 40 plus years...I'm sure the 'old boys' at BSA would have been very happy about that...Ian
email (option): ian@wright52.plus.com