ACW is one of my favorite periods and over the years I've been able to paint up approximately 25 regiments per side, plus guns and dismounted cavalry. Haven't gotten to my mounted cavalry yet. Since you're working with ACW which have fairly simple uniform colors, you might try an assembly line approach to speed up the painting job.

After you prepare and prime the figures you want to comprise your regiment (16-20 figures), line them up on your painting table in front of you. You might consider gluing them 3-5 in a row to a painting/popsicle stick with a drop of white glue for easy handling.

Then select your palette of colors. Your basic Union, for example, is easy. Dark blue jackets/overcoats. Sky blue pants. Black and/or brown for belts, canteens, and blanket rolls. White for gaiters and canvas bags. Dark brown rifle stocks. Silver or iron rifle barrels, fittings, bayonet. Flesh for skin. Black, brown, yellow for hair.

If every figure is similarly equipped, then start your assembly line with one color and paint one feature...typically the largest area of your figure or the most difficult to reach with your brush. For example, you might start by blocking in the dark blue tunic or overcoat. Do all 16-20 figures. Then pick the next feature and color, and repeat. Leave the finer detail for last.

You'll find the paint will have dried at one end of your painting line by the time you reach the other, so you won't have to wait. The repetition of brush strokes also really speeds up the painting process. Plus, since you are blocking in large areas first, it doesn't matter if you get a splotch of blue paint on the gun barrel or on the hands. You can clean that up later when you get to the detailing stage.

This basic blocking technique paints good wargames quality figures in 15mm and will get you started at 25mm, although you'll want to spend more time in the detailing and dry-brushing stages at the end. If you want to make them even fancier, then go back and try dry brushing or washes to create shading effects and give depth to your figures. To dry-brush, just take a bit of the base color and lighten it. For example, mix some of your dark blue tunic color with a bit of the light blue used for the pants. Take an old or frayed brush and dab it lightly into the paint. Then flick the brush against your fingernail or on a paper towel until almost all the paint is off the brush. Then lightly flick the brush across the figure you are drying brushing. It will leave a bit of lighter colored paint on the high points of the figure, making the folds and creases of the tunic (or pants, or face, etc.) stand out under close inspection. Another approach is to do a wash, either with inks or with a darker paint diluted with water and perhaps a drop of dish soap so that it flows into the crevices. Some folks with do a base coat, then a wash, and finish with a dry brush to highlight.

The assembly line approach is a little more complex if painting ragged rebs or yanks due to the lack of uniformity, but still works and will speed up your painting process. Just choose 3-4 different colors for pants, for tunics, etc. instead of one and then determine a desired frequency (e.g., 2 brown for every blue pants). Then work down your assembly line as described above, skipping figure based on your desired frequency (e.g. brown, brown, skip, brown, brown, skip, etc.). Then switch to the next color for that feature and start again with the figures that have been skipped.

If you want to try some fancy detail, like a checkered shirt or blanket roll, save that for last. The assembly line approach requires some discipline, in that your regiment doesn't really come together until the last couple of passes when you add the flesh and the final detail. But with this technique, I can usually paint a regiment of 20 15mm figures (5 stands) in about 5-6 hours.

Another approach could be dubbed "speed-painting". The key here is to remember that you only need to paint what you would see on the figure at arms length (i.e. the typical distance you would look at figures on the gaming table). Start by priming the figures in an appropriate base color (blue or grey). Give them a second priming coat when the first dries. Paint on the face and a different hat/hair color (keep it simple; e.g. all black or all brown), and paint the gun black or dark brown. Don't worry that your uniforms are uniform...not all rebs were ragged.

When you've got all the paint you care to put on your figures, protect your paint job with a matte sealant. I prefer to spray it on rather than try to paint it on. Some folks prefer a clear flat or gloss sealer (gloss supposedly dries harder and protects your figures better). Some go as far as to give the figures a gloss coat for protection and then a flat coat to cut the glossy shine.