Skin boats are typically coated with either single-part or two-part polyurethane, and that seems to be the case here. The skin material appears to be either Nylon on polyester, which is good, as they don’t rot. Where the finish has flaked off, it should be possible to re-coat it after it’s thoroughly cleaned, but the duct taped area concerns me, as that’s usually a sign of an emergency repair of a cut or puncture. If that’s the case, it will have to be patched, but that’s not a huge job. You’ll have to remove as much of the damaged finish as possible with a dull metal or plastic putty knife, with the corners rounded so they can’t dig into the skin. Any intact finish that’s well-adhered will need a light sanding in order to get a decent bond when you recoat it.
Also, I suspect that the boat has been stored outdoors in the sun, which is why the finish is so badly damaged. If so, it’s also means it’s been exposed to fresh water (rain, snow) and that can cause the frame to rot. Examine it carefully and push on the areas where the frame touches the skin to see if the wood is soft. If not, it should be OK, but if it’s spongy, walk away.
Assuming that this seems like a decent candidate for recoating, it may be possible to bleach some of the stains out it before you do. Sloshing a bleach and water solution around the inside would also kill any mold or mildew.
When you’re ready to re-coat it, you have choices of finishes. Minwax Helmsman Spar Varnish is one of the least expensive options and works well, but just be aware that it has an amber tin to it, in case that matters to you. Lowes sells it, but apparently Home Depot sells Varathane now, which I haven’t tried, but I don’t see why it wouldn’t work. I used Zar Exterior Satin oil-based polyurethane on most of my skinning jobs. It’s good stuff, and is quite durable, but it can be somewhat expensive. Apparently, Home Depot carries it now, so it should be easy to find.
You want to use oil-based varnish on synthetic fabrics, not water-based, so make sure you have a well-ventilated area to do the work. It will take 2-3 quarts to get 3-4 coats on the skin. FWIW, I think satin varnish looks best, but if you prefer gloss, go for it.
You could use a two-part finish, but they’re expensive, more complex to apply and I can’t see the point it doing it on an old skin.
The easiest way to apply the finish is with a small foam roller. Use a foam brush to tip off any bubbles, if necessary (Google “roll and tip painting” for more info). Keep a wet edge and don’t over-roll it.
That’s about all I have for you at the moment. If you have any questions, post them here and I’ll do my best to help.
