At the risk of being too long winded here is some waterproofing information from 1830.
WATER-PROOF DRESSING FOR SHOES, &c.
TAKE a piece of Indian rubber, about the size of a walnut.: cut it in sman pieces, and put it into a phial with four ounces of highly rectified spirits of turpentine. Cork it up for about a fortnight (more or less, according to cold or hot weather), and shake it every day. When this mixture has come to a consistence about the thickness of treacle, it is fit for use. You may then work it, with a paint brush, into leather, rope, or what you please. But, when used for the soles of shoes, leather trunks, or any thing that does not require flexibility, you should add, to this composition, three times the quantity of copal varnish. The most effectual mode of application is to anoint, not only the outside seams, but also the whole inside or the soles.
If you want this dressing in a hurry, and an extra expense is no object, you will find that ether, or reaptha, will dissolve Indian rubber, and dry, much quicker than spirits of turpentine. The powder colours, for painting, either with or without oil, will mix perfectly well with this composition.
The foregoing recipe was given me (just in time to publish it, but too late to make a proper trial of it), by Mr. Cornelius Varley, who tells me that he sent it, many years ago, to the Philosophical Magazine. Not wishing, however, to enter it without some kind of investigation, I applied to Mr. Fisher, the celebrated chemist in Conduit-street, who was good enough to make for me as many experiments as the limited time would admit of. The preparation which he found to mix the best, was three oz. of Indian rubber, boiled for about three hours, in a pint of linseed oil*, which thus became immediately of a good. consistence; but it required such a time to dry, that he afterwards found it necessary to add spirits of turpentine. In short, it has long been known that the solution of Indian rubber is a valuable recipe for making things waterproof. But there are so many ways of doing it; and, perhaps, among them all, the best not yet discovered, that we must, for the present, dismiss the subject by merely giving the hint, with the hope of putting our speculators on the scent, to bring to perfection what would be to their own advantage, and worthy the notice of the public.
* This comes very near to the recipe given in our former editions, and now very much in use, for dressing Russia duck.
Personally, I'd just use a can of Scotch Guard on the cape and lower parts of the trouser legs and call it close enough. It won't be obvious that you did and it will give you some extra water protection. As said earlier, dressing in layers underneath goes a long way toward mitigating rain and cold.