I spoke with the curators at SANHS about this,...
I spoke with the curators at SANHS (Springfield Armory National Historic Site) about this, and the moment I mentioned 'leather care' his eyes rolled back. Knowing about Renaissance wax and their efforts to preserve our National Treasure of a firearms collection at SAHNS (BTW; as the National Park Service doesn not have the funding to do so, the effort is currently being assisted by your dollars in the from of a yearly grant from the GCA to continue their efforts to preserve the collection housed there ...), I asked what professional curators do in museums to preserve the leather artifacts. He said there is nothing that can be done to stop them from degrading and eventually ALL leather artifacts will change and degrade. I got specific and asked about Pecards and the other leather care products available commercially, and all he would do is reiterate; there is nothing that can be done to stop them from degrading and eventually ALL leather artifacts will change and degrade to nothing but a pile of dust.
Once the cow died the leather started degrading and decomposing. Tanning turned it into a product we could use but it is hardly stable or permanent, and there is nothing that anyone or anything can do. He added that many of not most of the products out there will actually have a negative long term effect and be detrimental. He then stated that the ONLY thing we can do is keep them in as perfectly stable an environment as we can; not too dry not too humid, not too hot, not too cold, and above all; as perfectly stable as we can manage between all these variables.
That is it.
The result for me is that I do not bother trying to 'collect' the leather accoutrements and such - I know that I cannot do anything but watch them go downhill, inexorably.
Sorry if that is not the answer that you want, but my experiences with Pecards have, like Rick's, been wholly and completely unsatisfying. The white stuff never goes away, not here in the midwest, and it is an MF'er to get back off when it shows up.
Well you can do that if you want.
The point is that it is pointless.
For those who want more, when it comes to the conservation of these items there are a few rules that conservators go by. First; do no harm, and second; don't do anything that cannot be undone.
Arguably, all of these products (including Pecards) break both of these rules - they definitely cannot be undone and and many will actually have a detrimental long term effect and do not extend the life of the item but shorten it (though they make the item more useable while it is around - meaning anything that softens the leather is beginning to break it down ... you can add as much 'wonder-goo' as you want but you cannot rejoin those fibres and rebuild those cells ...). Also, like Pecards, they change the appearance by darkening the leather - irrevocably. On top of it, then to have all of that white crud reappearing over and over again (that even the most ardent supporters will grit their teeth over) is just frustrating.
The only thing that the curator at SANHS could give me is to keep them at a consistent humidity and temperature - cool and dry - and definitely do NOT get them wet, and then hope for the best. In a stable environment they may last for generations, while in the worst they may not even last half a year.
If you have already so treated some of your items with these products then it is what it is - I wouldn't use more. If you haven't, I wouldn't. I do nothing to any of them that I have any more. If you really want the things on your rifles or to own some of the other leather items, then make sure you get the best ones you can and then take care of them (as above). But do not grab a cheap one and think you can bring it back to life because that is EXACTLY what you will be trying to do (and if you buy into some of the sales-pitches, thinking you are doing ...) - bring it back to life - and unless you have some connections to the Almighty that you haven't clued us in about, you can't. :D This is the same across the board for all leather products - slings, boots, leather scabbords, leather cheek pieces, B3 bomber jackets and pants, you name it - the leather has a limited lifespan and use will use it up. But now who is going to use an original WWII sling in competition, anyway - you guys have nice new ones for stuff like that.
Other than that, :surrender:
I know this isn't what anyone wants to hear but all I can do is apologize for bringing the bad news - sorry.