While the Main Hull is upside down, I think it's a good time to get most of your coatings/paint on it, and even all of it on the bottom surface ;-)
First a few words on finishes and paints etc.
There is not much doubt that epoxy is generally harder and more resistant than polyurethane. (Polyester gel coat is generally even harder than epoxy but more brittle.)
One of the few disadvantages of using epoxy as either a resin or paint, is its failure to resist sunlight—or UV (ultra‑violet) light. This means that topsides MUST be overcoated (typically with 2 coats) with something more UV resistant. Decks are particularly vulnerable and even a UV varnish will rarely survive more than one sunny season.
As you cannot successfully put epoxy over polyurethane, you need to get all the epoxy coats on first, so consider this in your coating plan. You can actually stay with epoxy all the way for the bottom (underwater) but all surfaces that will be exposed to sunlight for extended periods will need to be overcoated with UV resistant polyurethane (paint or varnish), as already noted. Paint is clearly more resistant than varnish, so the price of seeing that lovely wood grain is that you'll have to re‑do the surface 2–3 times more often than if painted. Organizing to keep the boat under some form of sun cover, is another solution to consider, if you want to keep it 'bright'.
As epoxy is harder and generally more resistant than most polyurethanes, I would use epoxy products to get at least 2 undercoats, before finish coating with a polyurethane of your choice. These undercoats can simply be epoxy resin with an added pigment (grey and white) and I'd suggest to use one of each so that you can see where you are at when sanding.
Special sanding/pre‑coat epoxy products are also effective for achieving a final fair surface but these need to be applied not too thickly or final thickness can become too uneven.
Although typical coating applications are by a thin foam roller with the rolled surface 'tipped' (leveled) directly afterwards by 'a follow‑up person' using a foam brush, I have personally found that on flat surfaces, applications similar to that described under my article Sheathing 101, also work well with pigmented epoxy and/or paint and save both paint, time and the scrapping of rollers and brushes. With some practice, the right squeegee and a developed 'touch', one can get good coverage with thinner coats than normally resulting from a roller and while this may well mean you need to do 3 coats instead of 2, your end result will likely be better and your paint better cured, especially if using one‑part polyurethane that needs to evaporate solvent and air dry. (One‑part polyurethane applied too thickly takes for EVER to dry and if overcoated too soon, will always feel soft.) If you decide to try my squeegee suggestion, try one about 5–6" (125–150 mm) wide and slightly relieve the outer corners so that they do not dig into the paint. Also, slightly thin the paint so that it spreads well and the more vertical and firmer the squeegee is held, the thinner the coat resulting.
I hear the question—what about spraying? Because all these paints use solvents that are dangerous to varying degrees, I'd strongly recommend against it unless you really have a proper paint booth with forced ventilation and use full masks. (If you want to understand more about this, I recommend you start by reading the chapter on Safety, in the book "Gougeon Brothers on Boatbuilding".) An acceptable, quasi-spray-like finish can be achieved with careful preparation and various hand applications noted above, with far less risk to your health and the environment in general. The boat shown here, while not perfect, was finished using just a squeegee and never saw a brush ;-)
As far as what products to use, this will depend on where you are located and whether you dry sail or not.
For the bottom, I lean towards first using resin and micro‑balloons to achieve a fair surface in way of glass tapes etc and then 2 coats of epoxy resin with different colored pigments, sanding in between with an orbital polisher fitted with a 100 grit sanding pad to get a really fair surface and after a final sanding with a wet 180 grit, coating with epoxy with added graphite powder (WEST #423 or equal) for a fast, dry-sailed boat.
It's important to wash/wipe down with a wet cloth after each coat of epoxy, in order to make sure there's no greasy film or bloom called 'amine blush' left on the surface. This sometimes appears as a by-product of the epoxy cure and can reduce bonding as well as clog sandpaper.
(WEST System people point out that this occurs more often when temperatures are cooling off, so the use of a faster hardener can help avoid this.) A quick wipe-over with a rag and solvent (such as acetone) can also pick up moisture and other contaminants, but allow this to evaporate for 20 minutes before overcoating. A wipe-over PLUS sanding, is the best assurance for a good bond if the previous coat has fully cured.
For the topsides, I would recommend the same initial preparation and fairing as for the bottom, but with 2 overcoats of two‑part linear polyurethane paint (Awlgrip® is the most well known though other brands can be equally effective.) One major issue I found with this 2‑part paint is that the solvents used are extremely strong in odour and no doubt, a potent health hazard too.
However, another interesting option is a new 2‑part paint by the epoxy people SystemThree® which has the same linear polyurethane chemistry but uses, somewhat amazingly, water as a base and cleaner. Apparently there is far less odour so this may be a good solution. Price looks attractive by comparison and they claim equal endurance. Time will tell—but it's well worth looking at. Check out www.systemthree.com/store/pc/WR-LPU-Topcoat-c29.htm. It seems there are 12 colors but in semi-gloss only. Probably a function of the water base. They do offer a clear gloss though, that should be more durable than varnish. The only warning is that waterborne paints have a tough time in hot and dry conditions, so work best in humid air and also need super clean surfaces to bond with—sand or use an alcohol rag—nothing oily.
One‑part polyurethane is still a less expensive option, but will typically have less than half the life as this paint air-dries, compared to a 2‑part that chemically cures giving a much harder surface. Examples of one‑part marine polyurethanes are Easypoxy® from Pettit Paints and Brightside® from International Paints. There are others, of course, and I'd certainly recommend the Dutch company Epifanes that also makes some fine marine paints, though note that the quantity in each can is sometimes significantly less than the US standard quart (0.946 litre).
Before turning over the main hull, I'd recommend to totally finish the bottom and to get the sides to a near‑finished state, with not more than one more coat left to apply when the boat is up the right way and all the parts (beams and amas) are assembled. If the boat is to be left afloat for long periods, then a low friction antifouling will be needed and I recommend the VC17 (or equal) that is very thin and which can be applied even months before going in the water. At least do the flat of bottom that is hard to get at once turned over—the lower sides can be done later after a waterline is marked. 'Dry sailing' directly off the trailer and not bothering with this messy, expensive stuff is the best solution though for a small boat under 300 kg or so.
Once all is hard and well cured, it's time to turn the main hull over. At this point, it should only weigh 120–130 lbs (55–60 kg), so this will not be difficult. Cut the hot glue away from the bulkhead supports and once free of the building platform, slide the boat towards one edge. This will then allow the boat to be laid over, first on its side (on some foam to protect the paint) and then, the whole boat can be tipped once more towards its bottom while being lowered on to ground blocks fore and aft, with foam over them. This will place the hull lower than the table, so that access to work inside will be easier. The Build Platform will then make a low but useful bench.
Although we certainly spend more time working on a fine exterior finish, it's mostly for cosmetic and performance reasons. By contrast, the interior finishing is mostly for boat life and survival, so it's even more critical to make the right decisions. The MOST important thing is to seal all joints and fillet all intersections where moisture might collect. While it's clear even structurally that inner chines and bulkheads need fillets of at least 10mm radius, even a flat stringer laid against a plywood skin can greatly benefit from having a small (3–4mm rad) fillet at the skin intersection. This will permit whatever coating is then applied to be able to follow the 90° change of direction without a really sharp corner that would otherwise crack almost any coating due to the working strain and changing temperatures almost all boats are subjected to. So the first thing is to fill all the crevices and prepare all surface interfacings, so that follow-up coatings have a fair chance.
Once that's done, one can think about the interior coating options. Two coats of epoxy, with the last one being a fuller 'flo coat', will give pretty good protection for most surfaces and if these surfaces are in basically enclosed spaces away from regular sunlight, no further coating is required. If these areas are low in a boat that will likely have bilge water over it much of the time, then I would recommend to add a coal tar epoxy coat.
(All boats should have limber holes to permit full drainage of all areas except fully sealed tanks. Properly draining, drying and venting any boat after use afloat is a VERY important part of good maintenance. Washing off salt water helps too.)
Underdeck surfaces should have been pre-sheathed with epoxy and cloth and if so, one additional coating of epoxy is generally sufficient, but again, make sure that all corners are first filled so that there are no open crevices. Not doing this is the main cause of deck failure for boats (particularly amas) that are stored upside-down, so make sure you use enough filled epoxy when laying on a deck, that a bead of it will be pushed out and serve as a sealing fillet.
By far the lightest and cheapest finish for interior wood is a simple wood preservative and I consider pigmented aluminum paint to be very effective for that job. I've seen plywood boats coated only with 2–3 such coats, that still looked internally healthy after 25 years of service. By contrast, I have seen boats of exactly the same design that had 3 coats of epoxy, where the plywood had totally failed and the boat was scrap after 15 years. The reason was that the joints were badly made AND, there were no sealing fillets. This permitted water to get behind the epoxy film where it could never dry out and rot soon started. Even those bad joints would not have been such a disaster IF all had been properly filled and filleted. See also the Frame and Plywood article.
The two disadvantages of aluminum paint are first, that one cannot epoxy anything to the surface after it's applied without a lot of work to remove all of it and secondly, that it does not look particularly attractive. But for interior surfaces that typically hold no standing water and are mostly out-of-sight, it's an economical and lightweight option.
(Back in the 50s there was a craze in the UK to finish the underwater exterior of plywood racing dinghies with aluminum paint. The resulting matt finish was considered to be 'faster' than options of the day and to my knowledge, this has neither been proven either true or false.)
Otherwise, epoxy paint will work fine if the area is out of sunlight or use polyurethane for open areas such a cockpits. Larger boats that will be slept in, will benefit from the use of paints that can absorb condensation, but that will not apply to most small boats such as the W17.
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