For leather soles, once the blades lie flat enough to the sole that I am not having to create shims (or if I needed to sand any part of the sole down, I would have done it first, plus I would have identified the location for the four temporary mounting screws and would drill them in advance. I use SnoSeal for the next step. It seems better than any other product I have found because it contains more beeswax. Or some other kind of magic. There is also a family connection to the SnoSeal brand, but I will not go into that here...
I start by applying SnoSeal with a heat gun until the leather has saturated. The leather usually gets considerably darker, but to me it looks nice (not so dry and thirsty). The heated sole will soak up the SnoSeal for quite awhile--I treat one sole, then the other, and go back and forth until I feel like they are done (probably at least 10 times, sometimes more). The last coat of SnoSeal should be applied so it does not all soak in but leaves a little film on the surface. This can be polished with a soft cloth to act as a finish coat. Then I spread a layer of SnoSeal on the soles where the sole plates are going to sit, but leaving a 5mm-ish circle around the screw holes so I can insert the screws without coating them with SnoSeal. I line the blades up and tighten the screws. SnoSeal will be squished out around the edges and at the center of the blade (unless they have solid sole plates), as well as out of the other screw holes. This is a good sign that there is a nice layer of SnoSeal between the sole plate and the sole. I wipe off the excess but I leave a little in each of the empty screw holes to keep water out.
After any adjustments have been made to the mounting alignment, I remove the SnoSeal from the holes that will get additional screws and add screws. If the skates are for an adult, I have them test the alignment and usually will get back to gluing the screws on the next sharpening.
If the skates are for a kid (sorry, kids, no slight intended, but your feet are growing and you don't usually have orthotics in your skates, so there is a little more wiggle room for your foot to find the right place in the boot), I go ahead an glue the screws in at this point.
I have had better luck with a bit of clear epoxy on the screws than with silicone, but perhaps I used the wrong kind of silicone. Standard 5-min epoxy will adhere the screw to the leather enough that the screws have a tendency to stay put. It does not take a tremendous amount of effort to unscrew them if you need to--and there has never been leather stuck all over the screws. At first, I was really careful not to get any epoxy on the sole plates and to try to have the application of epoxy be invisible. Over time, I have discovered that it is more useful (and works better) to put a little bit of epoxy under the screw head and let it be slightly visible that it is there. Mainly it is a reminder to me as to whether I have or have not yet applied this epoxy.
If the SnoSeal is renewed periodically, I have had boots last for years without any water damage to the leather soles. Until I started adding SnoSeal under the sole plates, I used to see black growth there when I would replace blades, but I also was doing less adjustment (shimming or sanding) at that time, so the sole plates did not lay as flat on the sole.
All that being said, I live in a cold climate with a cold rink. There is no need to worry about skates getting hot and having the SnoSeal melt and ooze out from under the soles. I think if I was in a hotter climate or if we skated at a mall rink instead of in a barely heated building, I may want to try other options.
I have seen soles attached with silicone and have tried it as well. It should not be applied until the blade alignment is permanent because you will not be able to slide the blade sideways once the silicone cures. It is great in that the blade is solidly attached, but it is not something you want to try to undo very often. It can be used to help fill small gaps, though, and I have considered putting the silicon on the sole plate and then covering it with plastic film, then attaching it to the blade. This would allow it to form a shim that would seal the gap between the boot and blade but would still be simple to remove and you could also slide the blade over a tiny amount if needed for some reason.
I also tried sealing the edges of the sole plates with silicone in an attempt to keep water from getting under the sole plates. It just peels off. It seems to work fine in locations where it is compressed between two surfaces. Again, maybe I was using the wrong product.
Has anyone tried putting a bead of silicone around the sole plates with success (meaning that it is still in place months later)?