There is not really a "coastal. From Lisbon to Porto.
The Coastal leads from Porto to Santiago
You can follow two routes out of Porto. One direction Maia-airport Vilarinho to São Pedro de Rates .you walk in a very busy area with not a walkers friendly infrastructure alongside busy roads on the hardshoulder, heavy traffic touching you almost and cobblestone streets.
The other one is following the Duero river out of Porto to the Atlantic ocean and on to Matosinhos from where you follow a relaxed boardwalk alomgside the beach Direction Vila do Conde. From there you can go either follow the waymarkers to Sāo Pedro de Rates and follow the interior caminho Portugues to Barcelos, Ponte de Lima and Valença do Minho /Tuí. Or head for Póvoa da Varzím and Esposende to Viana do Castelo and Caminha alongside the coast where you can choose either walk on to Valença do Minho on the interior route or take the ferry in Caminha and head on to a Guarda, Oia ,Baiona and Vigo to Redondela where you hit the interior route again direction Santiago.
If you like the social side of the caminho choose the interior route-don't forget to stay at
Casa da Fernanda in Vitorino dos Piaës between Barcelos and Ponte deLima. Here you meet other pilgrims and it is a great place to stay.
If you like peace and quietness, go for the coastal caminho and if you get fed up with it after some "lonely"days you allways can hit the interior somewhere as mentioned above. Have your map or google maps ready to find all the places I described.
Bom caminho