Actually Hamilton House Silver Secret is a spot chemical plater (rub on in a few applications) which I have had good results with. Indeed the thing about the chem platers is they do not 'build up' the plating. They simply adhere to (well-cleaned) bare brass and the repeated applications are done simply to get all or most f the brass covered.
BUT...to my surprise...once done....it is more durable than I had expected. I actually did a neck with it once, owner didn't wanna spring for a true electroplate silverplate job....and the neck, once done, actually stood up to a light mechanical buffing without the plating buffing off. So, IMHO, it's OK stuff...I mean, it is cheap, easy, and it does have some resistance to wear.
I confirmed this when I used some on some palm key touches which had worn to bare brass. 3 months after the new owner (practiced and played around 4-5 days of the week) had taken possession of the horn, he sent me a pic and the touches had not worn the chem plating back down to bare.
In areas where there is not regular contact, I'd say it is a pretty cheap roll of the dice to give it a try. If it didn't work, you are out around $15.
The one aspect I will say about it is, in some cases (not all, but some) even after repeated applications to an area, there is still a brassy hue to the silver....it isn't as 'cool blue' a hue as an old silverplated horn would have. But, again, it did give a noticeable improvement in most cases where I have used it.