You could put them in a pan of boiling water for 3 or 4 minutes (not so to they're cooked through) before putting them in the oven.
The potato on the outside of the wedges will soften and cook a little bit which will help the outsides crisp up a little.
Make sure you allow them to drain completely before you put them in the oven.
cooking.stackexchange.com/users/5449/Red Spatula
After boiling for a few minutes in water, drain, put the lid on and shake (but gently so the wedges don't break).
This will rough up the surface.
The resulting bumps can heat through more fully and lose more moisture, leading to crisp.
cooking.stackexchange.com/users/8315/jontyc
I would also agree with other people's suggestions of par-boiling for 7-10 mins, followed by a shake.
In addition I would get the oil hot before adding the potato.
Either put the tray with the oil in the oven a few minutes before cooking or pre-heat the tray & oil on top of the hob before transferring to the oven (make sure your baking tray can withstand hob temperatures).
cooking.stackexchange.com/users/2389/Gary
I usually par-microwave the whole potatoes, then cut them into wedges and let them steam out for a good few minutes.
You can actually see the potato going whiter and drier near the edges.
Microwaving seems to me highly preferrable to boiling at this stage to minimise water.
Then I shake them in a bag with some olive oil to coat, then sprinkle with Caribbean seasoning.
Then bake.
Baking long and lower works better than too high a heat, too.
I've had them get so crispy before, they are almost like shards of glass, it's awesome!
cooking.stackexchange.com/users/58952/Steve M
Cook them hotter.
I cook mine at 425 or 450 F, which is 220 or 230 C.
It makes a difference.
I toss them in oil, sometimes with salt and rosemary, and put them on a silpat baking sheet.
I keep them a little apart, trying not to have any touching, so the steam can get out.
They get browned, even blistered, and crunchy, like roasted potatoes do.
To be clear: I don't parboil, I don't coat them in anything other than oil.
The oven temperature and the rubbery sheet on the baking pan do the trick.
cooking.stackexchange.com/users/304/Kate Gregory
What everyone has said..
par boil them then season when cooled coat in corn flour and a bit of oil and bake at about 200.
If ur wedges are extra big reduce the heat but cook for longer to stop the outside burning
Also try and bake then spread out well on baking paper.
My sweet potato fries come put perfectly like this x
cooking.stackexchange.com/users/23272/Keri Eloise
How do I make my wedges go nice and crispy in the oven?  I usually season them with a cajun mix, add a tiny bit of oil, and cook them in the oven on fan-bake at about 200 degrees (celcius). They come out okay, but  not  amazing.  What can I do to make them come out a lot crispier, while still using a normal household oven? Would more oil improve their texture, or do I need to crumb them maybe?
How can I make my homemade potato wedges crispy?
cooking.stackexchange.com
<potatoes><chips>
cooking.stackexchange.com/questions/22971
cooking.stackexchange.com/users/9882/kmb64
Par-boil or microwave the wedges for a few minutes before shaking them, seasoning and coating in corn flour and a bit of oil, Keep them properly spaced on baking paper and bake at about 200. You can par-boil them, give them a good shake and then bake in a hot oven with hot oil.
Par-boil them, give them a good shake then bake in a hot oven. Get the oil hot before adding the potato and make sure they are well spread out. Microwave them first before baking will save on water.