This is a complicated subject, maybe worthy of some discussion.
In the first place, I'm not sure what "copper leaf" actually is. Gold leaf - the real stuff - comes in the form of extremely thin sheets. (Gold has the unusual ability to be flattened out considerably thinner than virtually any other metal. Real gold leaf is, I believe, about a millionth of an inch thick. Any liquid or paste labeled "gold leaf" is not the genuine article. Real gold leaf is also quite expensive. Various companies in the arts and crafts world make "patent gold leaf," which handles and looks almost like the real stuff - and costs considerably less. I've seen stuff labeled "silver leaf" and "copper leaf" in similar packages - but I'm pretty sure it isn't real silver or copper.
Real leaf (and, I imagine, real silver or copper leaf - if there actually is such a thing) is tricky to apply. It requires, as Mr. Smith described, some careful preparation in the form of sizing and adhesive. If the bottom of your model's hull is smooth (e.g., if the model has a solid wood hull) this might work. If you're dealing with a plastic kit that has molded-in plating lines and spike heads, though, I'd be concerned that the sizing and adhesive would blur the detail.
I've fooled around a little bit with gold leaf on ship models, and I've never really been satisfied with it. Getting it to stick onto fine details without looking dull or powdery seems to be quite a challenge. I know some modelers swear by it, but frankly I've never seen a gold-leaf-decorated model that really impressed me. (Exceptions: the old Board Room-style models, which are enormous.) I really like the look of modern metallic hobby paints better.
If what you're after is a bright, metallic copper color, I'm inclined to agree with Mr. Smith: there are easier, more practical ways to get it. The "copper" paints in most standard hobby lines (Testor's, Floquil, Humbrol, etc.) actually give a mighty nice finish. An alternative would be the adhesive-backed copper foil sold by Bare Metal Foil. I haven't tried it, but the company has a good reputation.
The question of scale accuracy is another matter. The color of real "coppered" bottoms is a popular subject for argument among serious ship modelers.
Recent research has established that copper wasn't the only metal used for the purpose. The actual material - and its color - depended to a large extent on the date. Copper sheathing (with genuine copper) appeared at about the time of the American Revolution. (During that conflict the British Royal Navy copper-sheathed its warships on a regular basis when they came into suitably-equipped docking facilities. By the end of the Revolution most British warships were coppered.) It was discovered, however, that copper was fragile; it wore out pretty quickly. It was, however, in wide use in European and American navies and merchant marines from about 1770 to about 1850.
In the middle of the nineteenth century metallurgists started experimenting with other, tougher sheathing materials. Contemporary advertisements and building contracts for the American clipper ships of the 1850s refer to "red metal" (copper) and something called "yellow metal," which was touted as being considerably more durable (and more expensive). "Yellow metal," it's been established, was a mixture of copper and zinc - which, according to the dictionary, equals brass.
By the 1850s, it seems, big ships had pretty much abandoned copper and switched to yellow metal. There were variants on the theme; the
Cutty Sark, for example, was sheathed with something called "Muntz Metal." It apparently was indeed made of copper and zinc, but in proportions that gave it a reddish tinge. (George Campbell's fine book,
China Tea Clippers, which I happen to have at hand, says Muntz metal appeared in the 1830s as a 50/50 mixture of copper and zinc, but that in 1846 the manufacturer switched to 60% copper/40% zinc. The latter concoction, presumably, is what was put on the
Cutty Sark.)
To open yet another can of worms, start thinking about what the stuff actually looked like. There are different arguments about that, too.
Either copper or brass, if exposed directly to oxygen, will turn a mottled, rather light green; brass, if left alone long enough, will turn black. Theoretically, the scouring effect created by saltwater as the ship moved was supposed to erode the surface of the sheathing, constantly exposing new, bright metal. I have my doubts as to whether that actually happened. Photos of copper- (or yellow-metal-) sheathed sailing ships suggest that there was quite a bit of variation in the composition of the individual sheets; in some old pictures of drydocked ships the hulls look like patchwork quilts. And various things stuck to the metal. A ship that had been in the water for several years would likely be trailing a considerable amount of seaweed - to say nothing of barnacles and other marine growth. An accurate reproduction of a metal-sheathed sailing ship that had been in service for a while would not be the sort of thing I'd want in my living room.
Over the years ship modelers have taken various approaches to the problem. One is to sheath the hull with bright, new copper (or brass) and either spray it with clear lacquer (to keep it bright) or leave it alone and live with whatever patina it acquires. Some modelers heat their plates with a torch first. (I don't care for that technique myself. I'm not convinced that heat accurately duplicates what those real plates went through.) Another approach is to paint the bottom with high-quality copper-colored paint and say to h
l with it. Yet another (which I happen to like) is to paint the bottom with a mixture of pale greens, greys, and browns, thereby at least suggesting the appearance of a hull that's been in the water for some time. Or, of course, one can avoid the whole problem by slicing the model off at the waterline.
One of the nice aspects of sailing ship modeling, to my notion, is that it leaves considerable room for interpretation and personal taste. There are lots of ways to deal with metal-sheathed hulls. I say - read up on the subject, find out what options are realistic, and take your pick.
By now I suspect albinooscar wishes he'd never made his original post. Sorry about that. This is interesting stuff - and pretty important to the appearance of a finished model. Good luck. It's a great hobby.