I'd sanded the topsides, expecting simply to add another couple of coats of paint, but noticed some small blisters. Normally, a sign of osmosis, but I wasn't worried about that because I'd already removed all the antifoul from the underwater parts of the hulls, and they are in fine condition. Half a dozen blisters in total I should think. Moisture meter readings fine. And a bit of sanding below the antifoul suggested that the layer of paint on top of the gel coat was probably epoxy barrier coat. I did my best to leave this as intact as possible. I intend to add a few more layers of epoxy under the water line, and then add Copper Coat.
So I wasn't wasn't worried about the small blisters on the topsides. I just sanded them out. But then they started weeping some brown ooze.
You can see brown patches around the holes in the paint. Well, I had to investigate further:
So here's some paint scraped away.Under the white gloss, there's a blue undercoat I can't identify, but it may be an epoxy high build coating. The brown stuff I suspect is car body filler. Wherever I found pox in the topcoat I found body filler underneath. This theory was shaken a little when I found the blue paint under some pox in one section of the hull, but after a little sanding, I found filler under the blue paint. I was curious that the only places on the hulls where there was no pox at all, was under the saloon. I wondered whether the heat of the sun on the hulls had something to do with it, but I now realise that those areas were never filled - just painted over.
If I ever wanted a coating on the topsides that was durable, I'd have to remove all the body filler. I'd have to strip away all the paint. After spending days scraping away the antifoul, this wasn't a pleasant prospect, so I went home to do a little googling.
A steamer used for removing wallpaper proved to be the tool I needed. I wish I had tried it on the antifoul. 15 seconds steaming is all the is needed for the paint to peel away in strips, not damaging the substrate underneath at all. Of course that high temperature could damage the fibreglass if I left the steamer on for longer, but 15 seconds is enough to peel the paint, leaving the hull underneath just a bit warm.
I've now removed all the paint from a hull and a bit - it will all be done in a couple of days. And the filler is easy to sand away.
My hulls were constructed in this way - the underwater sections were made in a mould up to the level where the hulls flare, 2-6 inches above the water line. The hulls above that level have been made by bending on flat panels. These panels have no gel coat on, and sanding through the paint and filler (being very careful not to sand below that into the glass) shows the laminate underneath to be really excellent. No air bubbles at all. No dry glass fibres. It seems to have been an excellent layup, and very fair. Naturally, where the panels have been joined, there needs to be some filling and fairing, and some of it has been done with epoxy fillers:
In 2-3 days, all the paint and body filler will be gone. The hulls look terrible, but what I am left with is excellent. I intend to replace some of the body filler with epoxy-based filler where it really needs it, but I have no intention of filling and fairing forever. A fellow with a cat in the yard spent two months filling and fairing, and his boat does look brilliant. But I want to go sailing. I'll fill and sand enough to give a really good surface to be painted over, but I won't be applying filler to large areas in an attempt to take out some of the waviness of the hull. It's only slightly wavy, only enough that a boat-builder or surveyor would notice. I'm more interested in the durability of the work I'm putting into the boat than trying to make the boat look flawlessly fair. Life's too short for that.
I'll make the topsides smooth and fairly fair, add 2-3 coats of epoxy primer, and then some International Brightside - that's what I'm stripping off at the moment, and I think it would have been fine if I've originally painted it on a good sound surface.