http://www.fluentin3months.com/portuguese-after-spanish/ Hopefully that's a useful answer to your question mate!

JD, I'm brazilian and my native language, of couse, is portuguese. So I can tell you my experience with spanish. I've never studied spanish but I'm able to understand near 90% of what is said or written in spanish. I've already what TV in spanish with easy and I read text to the college in this language. That happens because they really similar languagues in many ways. But the problem happens when I have to speak or write in spanish. I just can't, at least not without taking so many mistakes. The biggest problem for me, and I heard friends with the same issue, is the vocabulary: it's really similar but it is not the same. So, when I don't know a word in spanish, differently from other languages, I just start using my portuguese again. See? If you have more doubts or any questions contact me. Bye.

Basic grammar is almost identical, but pronunciation can vary greatly. Portuguese has many colloquial dialects, especially in Brazil. Some sentences may sound strange when translating them directly into Spanish. You can use your Spanish as a basis to help you learn Portuguese, but remember they are still two different languages that do not almost meet eye to eye.

I am currently studying Portuguese and already am fluent in Spanish. I decided to start be focusing on pronunciation. I did this because reading Portuguese was much easier than hearing. Some words are the same they are just pronounced very differently. So learning the pronunciation can speed things up because you can recognize the similarities in the spoken. The thing I have found hard has been not thinking in Spanish when trying to respond in Portuguese!

I can help you if u want