Kevin:
I agree with Donald - especially the initial cleaning. I have used both enamels and acrylics without issues but always give the one coat a few days to dry and I also tend to use Dullcote after the paint dries.
I will add one recommendation. When I build plastic structures for customers I sand all plastic prior to paint. On stuff I can't sand I use steel wool. After paint I spray Dullcote to even out the surface in preparation for decals (some recommend gloss coat but I never had issues with Dullcote). You obviously can't sand the bridge but you can spray the bridge with Dullcote first to help adhesion and also between enamel and acrylic to avoid any potential issues. If you want a dull/rusty look the Dullcote will tone down any satin or gloss paint you use.
Joe
