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The more coats you apply, the more it will become necessary to scuff the surface to allow subsequent coats to adhere. That can be difficult if not impossible on some areas of a delicate ship model. Plus, if you don't get into all the nooks and crannies, they will stick out like a sore thumb.
A rule in the woodworking and furniture industry (in which I work) is that ALL wood surfaces, inside and out, be finished equally, lest uneven moisture incursion cause uneven wood movement. Moisture will migrate in and out of the wood with seasonal changes. If you don't think small pieces of wood can move much with humidity, just G00gle "weather stick". If you've built your model in the typical fashion, you have planked and decked it before any finish is applied. That means there is no finish inside, moisture will permeate faster on the inside than on the outside and cause uneven wood movement which could lead to warping and/or popped glue joints. The fewer coats of finish you put on only one side, the less this is a problem.
If you do use metal wool, use brass or bronze wool. Small particles of the wool will inevitably break off and lodge themselves in the wood grain. Steel wool will eventually rust and show up on your model, the others won't.
Finally, to see any glue spots prior to finishing, I recommend a solvent rather than water. As Bob mentioned, water will raise the grain requiring sanding. Again, this can be difficult on a model without losing detail. Use "real" mineral spirits, alcohol or acetone. I usually use alcohol because it stays long enough to see the spots, but still evaporates quickly. None of the solvents will raise the grain like water does.
Excellent points!
"The more coats you apply..." Coating buildup is to be avoided, to be sure. A common novice mistake is sanding
too much. It makes little sense to sand half or more of a coat off to provide a key for the next one. A flat finish doesn't require sanding for the next coat to adhere. Notably, shellac needs
no sanding to key the surface. The alcohol solvent softens the surface of the previous coat. The same is true to a lesser extent with "hot" solvent-based paints. Not so with water-based acrylics, however.
There are really two approaches required when painting models. You can sand to fair the "wide open spaces" such as hulls and it is essential that these surfaces are "smooth as a baby's bottom." Defects on a large smooth surface will draw the viewer's eye and destroy the impression of reality. On the other hand, the smaller areas, which is pretty much everywhere else, are impossible to sand evenly. There, it's best to forego trying to sand around portlights, hatches, and so on.
The rule is "more thin coats are better than fewer thick coats. Don't expect to get an even coloration without using a proper base coat.
"...ALL wood surfaces, inside and out, be finished equally, lest uneven moisture incursion cause uneven wood movement. ..."
Yes, all wood moves more than most people think. Shellac is one of the most moisture impermeable coatings known to man. Surprisingly to many, epoxy is
not moisture impermeable. Most coatings will slow the absorption of moisture into wood, but none provide total barriers. By coating the entire piece, as noted, the
rate of absorption is even, but the wood still moves. Wood swells and shrinks most
across the grain in most wood species and much less so over the length of the grain. Thought should be given to the grain orientation of wooden parts, lest this expansion and contraction differential in relation to the grain orientation can cause a model to bust glue joints or even split wood when significant changes in humidity occur.
I have found it worthwhile to seal
every wooden part of a model with thin white (clear) shellac (e.g. 2- or 3-pound cut) which soaks into the wood, does not build up and impair detail, and dries invisibly. Not only does this stabilize the rate of wood movement, but it also provides a good base for subsequent coatings. The alcohol in shellac evaporates very quickly, so there's very little time required for it to dry.
"If you do use metal wool, use brass or bronze wool. Small particles of the wool will inevitably break off and lodge themselves in the wood grain. Steel wool will eventually rust and show up on your model, the others won't."
Boy, it sure will! In my full-sized boatbuilding days, I used nothing but bronze woo. (I've never seen brass wool that I can remember.) I used to get it at the chandlery. When all the independent chandleries got bought out by big chains more interested in selling sunglasses and "yachting togs" than real marine hardware, I'd ask for bronze wool and get a blank stare, or else the kid at the counter would think I was pulling his leg. I'm sure bronze wool is available somewhere online now, but I sure haven't seen it on the shelf anywhere in decades. Like a lot of my colleagues, I replaced it with Scotch-Brite abrasive pads and never looked back. (The pads have the advantage of not filling your hands with tiny razor-sharp bits of metal, too!)
"Finally, to see any glue spots prior to finishing, I recommend a solvent rather than water. As Bob mentioned, water will raise the grain requiring sanding. Again, this can be difficult on a model without losing detail. Use "real" mineral spirits, alcohol or acetone. I usually use alcohol because it stays long enough to see the spots, but still evaporates quickly. None of the solvents will raise the grain like water does."
What a great idea! I wipe down workpieces with acetone after using a tack cloth, just to be sure I've got it all. It may be a "belt and suspenders" thing. I dunno. I didn't mention that because it seems a lot of people these days operate on the "if you can smell it, it's dangerous" principle. Alcohol certainly won't hurt you when used responsibly. A tack cloth is also an essential for any fine brushed or sprayed finish in my book.
Like yourself, I learned a lot of what I know about finishing furniture in a furniture factory. I my case, the factory was in a prison where I supervised the convict furniture factory workers.
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