If you use alcohol as a dye with color, what percentage alcohol is the best?

Does the higher content of alcohol give a better penetration, and do you need a sealer before applying a finish, and what finish is best over hardwoods?

When considering alcohol, your major concern is how much water is in the can. Cheap denatured alcohol causes the wood to whisker and raises the grain because of the water. Secondly is methanol used, because it absorbs into the skin so easily and is actually added to the mix to make it poisonous. They don't want people drinking it. Sealer and finish over hardwood is such a broad question, there is not really an answer. Some product is self sealing, most is not. Best finish over hardwood? Depends on what it's used for, where you live for environmental usage, if you have a quality spray booth to remove hazardous fumes for your employees, etc, etc..

When using a dye my preferred way is to tint an alcohol based sealer (aka dewaxed shellac). Ive found that this makes the dye much less likely to have lap lines etc. My preferred alcohol sealer is Parks Profinisher Universal Sealer. Hope this helps.

I prefer to use Mohawk M520 dye colors..... All of my finishers know the colors and can visualize in their head what colors they need to color match all the projects coming through production... the colors are predictable/consistent and we know their strengths well..... We typically use 4oz of M520 to 28 oz. of acetone for making color passes on raw wood and in between coats of finish for color tweaking..... some colors we dilute more... some colors we dilute less.... it's just an experience kinda thing... you have to know the products..... They are tools...... know when to use the right tool at the right time and you have a recipe for success....
dyes are not our everyday all day... just depends on the look we are going for.
We are probably 50% wipe with the custom Sherwin SB stain system 25% spray no wipe with Sherwin SB system diluted with VM&P Naptha and 25% straight M520 to raw wood....... Just depends on the control samples we are trying to match for the jobs coming through......

These are some really fundamental questions you're asking. I'd recommend reading some books on finishing. Bob Flexner and Jeff Jewitt have written a few good ones.

Depending on the look you're trying to achieve, dyes alone may not be the way to go. They're more often used as a base tone prior to applying a pigmented stain, or as a toner mixed into your topcoat to even out the color on wood with varying colors (like a mix of heart and sapwood).

Dyes offer no protection, sheen or sealing of wood, so a topcoat is required.

The "best" finish depends on so many things that it's impossible to answer this in a single post.

Cheers,
Pat

Pat's response is the best! A dye alone are for bright toys. I think the Original Poster has never actually been a 'finisher' per se, from the questions posed.