I had a bad experience painting a Stingray Submarine about 4 years ago for a friend. I painted the various blues and silvers onto the model using Halfords automotive acrylics, and finished off by airbrushing a gloss Tamiya acrylic varnish. When it hit the water for the first time, as it dried out after the run it developed light blotchy patches all over the model, most noticable on the dark blue. This resulted in a fair amount of cursing on my behalf. My intention had been to keep all the paint acrylic on the model. In the end I airbrushed a gloss ENAMEL varnish on the model, this proved to be waterproof and it maintained its gloss finish through many dunkings. I have also hand brushed gloss varnishes recently, as I find the process of thinning the paint for spraying introduces a slight matting down of the finish. Despite being a devout 'airbrusher' I find that you can still get quite spectacular finishes by hand brushing.
Good Luck with your restoration.