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Shalfleet
11-13-2007, 10:44 PM
I would like to refinish the bottom of my 1962 Thistle class dinghy and am looking for some ideas. She is made of moulded ply, is in pretty good condition and does not have many layers of paint.

I was planning to flip her and wetsand most of the paint off, especially the stuff that is flaking. As I race, I was also planning to fair the hull and wondered on the best filler. I have epoxy and microballoons which I think would work fine, but I'm worried about sealing in moisture so perhaps something more permeable would be better? One point to consider is that a few of the joints between the edges of the moulded veneers are slightly open, so the filler will need to handle any expansion and contraction that I assume would occur at these joints. A number of wooden Thistles have been fiberglassed on the exterior which solves some of these problems, but this is not an option I am considering.

I am pretty sure that the paint currently on the boat is Interlux Topsides which seems soft, so even 1000 grit wet and dry paper really rips into it. Wet sanding the bottom is something a lot of dinghy sailors tend to do, so perhaps a harder paint would make more sense, although this might be more prone to crack, especially at the joints between the veneers.

By the way, the boat stays in the garage and the most she will ever spend in the water at one time is a day or two.

Advice is greatly appreciated.

Dale

Martin Nelson
11-14-2007, 01:10 PM
First check what your epoxy supplier says about fillers. After sanding I would mix some epoxy and filler (microballons?), going heavy on the filler. The more filler (and less epoxy) the easier it will be to sand. Take a mixing stick and force the mix into the cracks. Sand after it hardens. I doubt that there will be a lot of expansion or contraction once the joints between the veneers are sealed. You might ask some other owners of wooden Thistles about paints to use.
Also, do a search for epoxy and fillers. There should be several posts about the topic.

Todd Bradshaw
11-14-2007, 03:43 PM
A couple points to make sure we're on the same page:

First of all, any fairing fillers should be applied to bare wood (all paint removed) that is quite dry and which has been sealed with a saturation coat of plain epoxy.

Expansion, contraction and water absorption will eventually destroy a laminated boat. The cracks between veneers that you are seeing are likely the first sign of this happening. The boat was not intended to take-up and swell the way a carvel-planked boat will. Anything and everything you can do to prevent water from getting into the lamination from both sides will likely be the best bet to help guarantee that you still have a boat ten years from now.

Fiberglass would add some weight (which you don't need) some abrasion resistance and extra stiffness (which never hurts, but if the hull is sound you could probably do fine without them) but more importantly, if you decide to seal the hull with epoxy the addition of a layer of light fiberglass can serve as a thickness guide to make sure that you have the required 10 mils or so of epoxy on the surface that's needed for a proper moisture resistant barrier (and can thus be quite handy).

Microballoons work nicely for fairing, but sanded microballoons are a lousy surface to paint over. Sanding actually breaks open a goodly percentage of the balloons on the surface, leaving little air-filled craters. Paint will not flow into these and they tend to yield a painted surface with a lot of tiny pin-hole defects (either immediately or after the hull heats up in the sun and the skinned-over craters start to pop). For this reason, microballoon fills should be sanded, overcoated with plain resin (which doesn't flow in there either, but which is generally strong enough to bridge the craters) and then final-sanded and painted.

There are racers who wet-sand and those who don't. I don't know what the current thinking is as to which finish slips through the water faster. In any case, it isn't something that needs to be done often and topside enamel should be OK. Once you have the desired surface texture it's more a matter of keeping it clean than for some reason needing to renew or re-sand it. Some hard-core racers will also wash the bottom after a race and then coat it with liquid soap and let it dry for storage. Before launching at the next race they hose it off and have a hull that's free from road dirt, etc. (works great in California - not so great in places where you might get rained on going to a race:))

At a minumum, I suspect that this boat needs to be thoroughly dried out, followed by filling the cracks and very careful coating it inside and out either with paint and varnish as it was originally (a base of CPES might be well worth the trouble) or with some sort of epoxy resin, followed by paint.

Wild Wassa
11-14-2007, 10:40 PM
Following the other comments;

Cutting and polishing the paint is what it is all about after fairing and painting. Then Starbright polish is the go, for a serious racing finish.

I cut and polish racing hulls to a good level. I'm doing a cut tomorrow on a Farr 6000 that I have refitted. The temperature is hot here, cutting surfaces is a good way to lose weight and get fit for the racing season. I normally cut the paint about 4 weeks after the paint was applied.

On a Thistle hull it should take you one long day's polishing per side after the cut to look good, the cut takes as long as a cut takes ... then there are the foils to cut and polish. I can cut and polish the paint on both sides of the foils in a day if I stick with it. I put a lot of effort into the foils ... the foils are the underwater powerhouses.

Farecla make an excellent range of liquid cutting compounds suitable for the different paint types. I'm a 2 pack water based polyurethane user when a racing surface is wanted on wood. I use Farecla G-3 Advanced Liquid Cutting Compound for cutting the polyurethane. Then copious amounts of Starbrite well polished, enhances the time consuming work. I use a Ryobi Pro polisher, and replace the sheepskin pads several times when doing a hull. I change the pads between the cut and polish.

When I'm using cutting compound, I extend the Farecla G-3 with lots of water sprayed on to the hull and foils, it gives an easier flow and a less sticky feel to the cut. The water extends the Farecla greatly, Farecla is expensive ... water is not as expensive.

The thing about making a racing surface is that it takes a goodly amount of time to get the surface just perfect before the painting ... a racing hull finish is 98% prep. If the finish isn't perfect, well enjoy being down at the back of the fleet.

There are no short cuts to making a really slippery surface ... it all starts way back at the clean wood and each layer as you work your way to the top, is in itself a racing surface.

If you're thinking about using International Perfection, a two pack polyurethane, you will read "do not cut or polish International Perfection." We always cut and polish International Perfection, it becomes twice the product. Perfection cuts and polishes just beautifully. It loses the ridiculous gloss when it is cut, but we want more out of Perfection than just a gloss. Perfection is an extremely flexible polyurethane, perfect on wood for non professional painters to use ... but I prefer Aquacote a 2 pack waterbased polyurethane for a more upmarket racing finish.

Other paints that I use for a racing finish are Imron and Berocryl ... if you are thinking about spray painting the Thistle. Of the paints mentioned, only Perfection is a brush on paint.

Good luck and remind yourself that hot racing surfaces don't make themselves.

I have an old English bay boat, a Coachwood Seafly, the racing surface on her is as close to slippery as I could make it. I didn't paint the racing surface so much as constructed a racing surface. Think along those lines.

Warren.