I guess up vs. down cut spiral depends on where your router is (i.e. handheld vs. table)...either way, I assume that you want to spiral to drive material into the work / substrate to prevent it from peeling up. Is this the correct assumption?
Thx
The terms can be confusing, but "Upcut" draws the waste *towards* the router and is a great general purpose bit for handheld and table mounted use. It also ensures that you always maintain full control over the router and workpiece because it is always pulling the two towards the router's subbase (or the router table's surface) so there is little chance of loosing control of the router. If you have dust collection, these bits are compatible with dust collection because they pull the waste towards the router's dust collection port or below the router table.
A "Downcut" is more along the lines of what you are describing, but downcut can create control issues if you try to take too great of a bite because it will be pushing the router *away* from the control surface (i.e. trying to physically lift the router). This can be especially troubling if something catches and you are not expecting it because there is a chance of loosing control of the router in a handheld operation. If you have dust collection, these bits are generally not terribly compatible with such as they are either pushing the waste towards the floor if handheld or launching it straight up into the air when table mounted.
Even so, I don't usually have an issue with tearout, even in difficult woods, when using an upcut spiral and if you do you have a couple of alternative solutions to address the concern: 1) tape the edge with a low-tack masking tape so that the tape provides added stability -- the router will cut clean through the tape; 2) Use some masonite (plywood, etc.) to make a zero-clearance subbase for the bit (drill the hole 1/64" larger than the spiral bit's diameter, or ream/file it out a little after drilling); or 3) use a thin piece of masonite (plywood, etc.) taped down to the veneer (you need only tape it enough to keep it in place and under control, no need for double-sided tape) -- the masonite will provide the necessary support to prevent tear out. If you should get very minor tear out, you can always follow up with a chamfering bit (or even sanding block at an angle) to chamfer the edges slightly, which would also make the veneer less prone to lifting when something catches the edge of the veneer in terms of normal wear and tear, so there is that recovery option if needed.
Typically, I find
lots of opportunities to make use of upcut spiral bits, but only rare opportunities where a downcut bit would be substantially more preferable. If you do opt for a downcut I would only use to remove the very last sliver of remaining wood and not for removing any significant quantity of overhang. There are also some
combo upcut/downcut versions (e.g. half and half) made specifically to try and manage both concerns (similar to some combo reverse/normal scrollsaw and jigsaw blades), but I have no experience with them and can offer no opinions.
Ultimately, though, it is your creation and you should base your decision upon what you believe is best as I'm not the one that will exchanging expletives with the wood if something goes wrong!
Best of luck and we really look forward to the photos when you are finished!