I have never had any problems using Titebond 2 for any speakers I have built with MDF (including 1.5" t material) I have always tended to put veneer on afterwards as I genereally tend to screw something up prior to veneering. :-) This also gives you a chance to listen to the speakers prior to putting on some expensive veneers, only to find out that they sound/measure terribly, and you have to haul them to the dump or make mitre saw stands out of them. I have always used a straight cut bit with no problems. As far as preference for type of bit, well that depends on the finished look you are trying to achieve.
I am building a project in MDF. I had to face glue a two large 3/4" and a
1/2" pieces together. I used Tightbond II and screws. As for a router bit, a straight cutting bit is fine. Using a saw and especially a router on MDF throws up a TON of dust. I set up my work on some saw horses outside to do the routing and I wore a face mask (respirator) even though it was outside. In many MDF boards, the resin used to hold the particles together contains formaldehyde which is considered a probable human carcinogen so either wait for a big wind or wear a respirator with a filter suitable for this kind of material. I would opt for the respirator.
My MDF work is generally biscuited, so I use cheap white PVA in a roller bottle for the edges, and the same stuff thinned down a bit in a biscuit-nozzle bottle for the biscuit slots.
PVA isn't particularly price-sensitive, but it doesn't like excess storage. New cheap stuff by the gallon is better than uber-PVA that spent the last winter out in the garage.
Spiral is better, but not better enough to be worth the extra expense. Look at kitchen fitter's extra-long trimming bits. You'll be wanting an nice heavy and stable router if you're joint 1 1/2" thick MDF - but I guess you're actually doing this as staggered laminations of 3/4" ?
Definitely after. It's much easier to not veneer at all, by buying in ready-veneered boards. This is a good plan, possibly the best. If you're going to bother veneering at all, do it after and do it right.
For veneering glue, I much prefer hot hide glue. It's a pain to find a gluepot for it (I use an old kitchen slow-cooker), but the tack behaviour when you apply the veneer is much nicer to work with.
I never trim veneer with a router. If I'm fooling with veneer, I go very traditional on tools - veneer saws to cut it (sharpened to a knife edge), then low-angle block plane and glasspaper to trim it.
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