Any single pack alkyd will do. I got mine fresh from the factory at HMG in Manchester, I used the coach enamel last year but its pricey so years before that I ordered the Speedline coating, not quite as mirror shiny but I also paint plant equipment with it no problem, so it's more than tough enough for the rigors of a canal. They make the coatings for a few brands including one that sells it exclusively as paint for narrowboats. If you give them a call they will give lots of good advice on choosing a top coat. It's important to check that your top coat will not react with the undercoat so either find out what it is you are painting over or strip it back as much as possible, realy you should do this anyway. It's highly unlikely you are going to come across an odd primer like cellulose or lacquer primer so If it can't be removed with a big tin of elbow grease then it's probably okay to paint over. Remember the prime purpose of the coating is to protect the steel from oxidization. Alkyd is very forgiving some more than others, Speedline is one thats forgiving but not the only one. Poor prep is the main cause of almost all failed coatings so get sanding and have fun!