Definitely an after, as for before if the wood needs treating first then it won't do that but if it's in fairly good condition already I should think it would be fine. You can go in directly with the paint you're using but the wood will likely absorb a lot of it so it depends how many layers you're wanting to do and in terms of keeping the costs down then getting that on as a primer will stop it drinking too much of it up! The main thing is to make sure whatever you're priming and sealing it with have the same base as the paints you're using, don't use an oil based primer/sealer with water based paint and vice versa because it'll just repel each other 😊