An amateur butting in W his .02 cents.
I'm no expert but, let me toss my .02 cents in here just for squints & giggles.
I think proper surface prep is a mandatory step in all areas of knife making, or painting for that matter but it's especially true when working W adhesives, I used to like to clean W lacquer thinner, I know that the proper type of alcohol will work well also.
I also only finely finished up to the area left uncovered by the scales.
(tang stamp area)
I found it helpful to get the scales and tangs true flat which I feel requires at least going to about 220 or so then I would go back and use a very narrow strip of 36 through 80 grit wrapped around a very hard, smooth, narrow
(steel or micarta) sanding block on the center areas of both scales and tangs being very careful to stay away from all the mating edges to make sure all the handle seams would mate flush against the seating surface, thus creating if you held your mouth just right a very fine join that seems almost seamless.
Having said all of that, I have still had a number of scales come off after 3 to 10 yrs of hard use, especially if I didn't use any pins
(Important step: flare hole & expand the ends of the pins), but I NEVER had 1 accidentally come off that I used bolts or revolver grip screws on even W/O any Epoxy, I tend to like my scales to be easily removed for routine maintainability or just a change of pace on fixed blades any way, so . . .
To the New guys:
Good solid advice has been given by others in this thread
(especially the old Dogs) but remember there's always more than 1 way to skin a cat.
When I do finally start makin again I will stay away from Adhesives as much as possible
(like the plague), but I just might give that HD loc-tite a try, after I test it out 1st. of course.
Good luck to all, Woof!