How can I say "Vou assistir o filme no final de semana e depois te digo o que achei",
      Would it be correct to say
    (a) "I'm going to watch the film on the weekend and then I'll tell you my impressions"?
    (b) How/what about "I'm going to see the film this weekend and after I'll tell you what I found about it"?
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    My versions now, as alternative ways, perhaps some are a bit more natural.

- I am going to see/to watch it (the movie) on the weekend, and I will comment on it then.

- I am going to see this one on the weekend, then I will let you know what I think about it.

- I am going to see this flick on the weekend, then I will tell you what I think about it.

- I am going to see the movie this weekend, and (I will) tell you my views/impresssions.

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       But then, answering your question: (a) is fine to me.

       As for (b) the use of "how about/what about" would be acceptable, tough

placing "after" there is not much natural to me.   

       Same to "found", that sounds like something you discovered by searching or studying (intently, of sorts).    Anyway, it could be, in certain contexts.

       Lets wait further comments, by more advanced learners, teachers or natives.