XI. Painting the Hull / Keel Guard
Building the Core Sound 20
June 6, 2008
NOTE: This article is divided into chapters. Click here for the Table of Contents.
After letting the epoxied hull cure for one week in the hot Georgia weather, we roughed up the surface with 80-grit sandpaper (to provide "tooth" for good paint adhesion), washed the hull with soap and water (to remove dust and the waxy blush that forms on epoxied surfaces), and wiped the hull down thoroughly with Interlux 202 solvent wash (to remove any remaining impurities).
To paint the exterior hull, we're using Interlux VC Performance teflon-impregnated epoxy paint. Intended for bottom-paint purposes, it produces a slick, hard, durable, stain-resistant finish that can stand up to the abuse that comes from riding on trailers or bumping against docks. Although it does not contain anti-fouling chemicals or ablative (shedding) properties, this won't be a problem for us, as we have no plans to store our boat in the water long-term (when marine growth becomes a problem).
Most folks use this paint only below the waterline, but we decided to coat the entire hull with it to avoid any color mismatch with the topsides paint we bought for the interior. Both the VC Performance and our topsides paints are white, but whites can be notoriously difficult to match, and we didn't want to attempt any custom paint tinting. Instead, we'll use our topsides paint solely in the cockpit. (Since the cockpit is separated from the hull exterior by the bright-finished (varnished wood) deck, any mismatch between the whites won't be noticeable in that case.)
The only other big painting decision we had to make was regarding application. After soliciting many opinions, we decided to spray on the paint using a high-volume low-pressure (HVLP) sprayer. (VC Performance can also be rolled on, but it's formulated for spray application and several people who have applied it both ways claim that spraying yields much better results.)
HVLP Advice
Skip this section if you're not interested in the technical details of painting, or don't need advice about HVLP sprayers.
If you research HVLP spray systems online, you'll discover a lot of contradictory opinions about the "necessary" equipment for "high-quality" HVLP paint application. Some folks claim you must spend at least $200-$400 to get a decent HVLP gun, while others claim that $50 guns yield outstanding results. Some adamantly recommend "true" HVLP guns that run on turbines, while others recommend "conversion" guns that run on air compressors. Of those who recommend air compressors, many claim that the compressor must be a minimum 5hp with at least 60 gallons of air capacity, while others insist that almost any air compressor will do, even if it is less than "ideal". A few even claim that "professional" finishes can be achieved with the basic $100 Wagner sprayers sold at Home Depot. The list of contradictions goes on.
We chose to ignore the recommendations about expensive systems because we have no intention of painting boats for a living. We figured that as long as the air consumption (cfm) of the HVLP gun is lower than the air output (cfm) of the compressor, we would be fine. So, we spent $90 on a Kobalt HVLP gun (model KC-577HG, at Lowe's), hooked it up to our $200 Campbell-Hausfeld air compressor (1.7hp, 20-gallon), and started spraying. Guess what? It worked great!
Sure, our compressor works hard (near-continuously) to keep up with the gun, but it does keep up because the air consumption on the Kobalt HVLP gun is an impressively low 5.8 cfm @ 29 psi (ideal for folks with small compressors like mine). More importantly, it lays down a very nice paint finish in less than a third of the time it would take to apply by brush and roller, coating the entire hull in about 20 minutes! Could I get better results with a high-end $200+ HVLP gun and a dedicated turbine or $1,000 air compressor? Perhaps. But not dramatically better and not enough to justify the higher expense. The $90 Kobalt gun produces far better results than I could achieve with a brush and roller, and that's fine by me.
Spray Settings
If you patiently tweak the gun settings and practice with it a bit before painting the real deal, it is perfectly adequate for this sort of job. The 1.5mm tip size that comes with the Kobalt HVLP gun meets the proper specifications for spraying VC Performance paint, and I found that a setting of 50 psi at the compressor and 40-42 psi at the gun (Kobalt includes a gun-mounted regulator) yields the best paint atomization. This gun setting is higher than the 29 psi recommended in the Kobalt manual and produces a bit more overspray, but the finish comes out finer and lays flatter.
Paint Volume
For the first coat, we mixed 24 oz. of base with 24 oz. of curing agent, allowed the mixture to sit for 30 minutes of "induction", then thinned it 10% with 2617N reducing solvent. This yielded enough paint to apply one good coat to the entire hull, as well as to the rudder housing and rudder blade. On the second coat, we mixed the same amount, but had about 6 oz. of leftover. So, for the third and fourth coats, we reduced the amount of paint to 20 oz. base / 20 oz. agent and 15 oz. base / 15 oz. agent, respectively (all thinned 10% with 2617N solvent), which again turned out to be ideal. Of course, paint results and volume needs will vary depending on the spray-pattern and fluid-flow settings, as well as variations in temperature and humidity.
Application Tips
Here are some important tips I learned while working with the HVLP sprayer:
- Make sure your paint area is reasonably sheltered from dirt, debris, and bugs that may get into the (wet) paint finish. We painted outdoors, but found that draping plastic drop cloth around the perimeter of the boat (and duct taping any seams) yielded excellent protection. While spraying, open up the area to provide good ventilation, then seal the area again while the paint dries.
- Wear a full-body paint suit and a respirator at all times while applying paint. Make sure the respirator is rated for organic vapors and paint spray. A dust mask will not protect you from atomized paint particles.
- Install a water/oil filter-separator in the airhose to collect condensation from the compressor before it reaches the gun. If the moisture gets into the paint, it ruins the finish. Even with a separator, we found it necessary to wrap a towel around the air fittings immediately below the gun because small droplets of condensation can occasionally escape through that connection and drip onto the hull.
- Clean the little spray filter located at the base of the paint cup before spraying. The filter must be kept clean to permit an even flow of paint for good atomization.
- Strain your mixed paint through a fine mesh strainer when pouring it into the gun cup. This will remove clumpy particles that would otherwise plug the gun.
- Test the spray gun on a piece of cardboard and adjust the fluid flow and spray pattern adjustments on the gun to yield an eliptical pattern about 6-8" tall with the smallest possible paint specks surrounding it (i.e., best atomization with good flow).
- Spray in smooth, even strokes, holding the gun perpendicular to the surface and moving your entire arm over the surface (rather than rotating your wrist, which will create uneven coats by rotating the spray nozzle out of perpendicular from the surface).
- Overlap strokes slightly as you go, working your way from top to bottom, end to end.
- Do not try to achieve full coverage (i.e., completely blank out any trace of the underlying surface) on the first coat. Spray in thin, even coats, and by the third coat, the paint color should "even out" to conceal any color variations or patterns on the underlying surface.
- Periodically check the paint flow by spraying a piece of cardboard to ensure paint is coming out. The pressure stream of air can look like paint, but occasionally, the spray filter at the base of the paint cup will become clogged and need cleaning. If so, empty the paint cup into a clean container, unscrew the paint cup, clean the filter, reassemble, and continue painting.
Paint Results: A Fine Finish
After applying six coats of VC Performance over a period of three days, our hull was finally ready for wet-sanding. Wet-sanding with very fine grits produces a glossier, smoother, more efficient hull finish. Since the VC Performance paint is impregnated with teflon, it also comes out extremely slippery, reducing surface friction on the water for a faster ride.
We started by sanding lightly with 220-grit pads on the random-orbital sander, taking care not to sand too deep. Then we switched to wet-sanding by hand with 400- and 600-grit paper. It's amazing how much a little sanding can bring out the glossy sheen of the paint. Once sanded to 600-grit, the paint was so smooth and semi-glossy that most people would not be able to distinguish it from factory-applied gelcoat.
Originally, we had planned to sand to 800-grit, but the paint was already wearing thin in a few places to show some of the color variations on the underlying surface (one of the downsides of a no-primer paint). We decided to leave the minor imperfections in the coat until the next time we re-paint. This was a reluctant decision for Wes since he's a perfectionist and was hoping for flawless results, but the finish is already much finer than what we've seen on many other homebuilt boats (including the last one we built), and it looks very good.
Stainless Steel Keel Guard
To protect the keel from abrasion and damage, we installed a full-length stainless steel rub strip. We used 3/4" half-round for the first six feet because the rub strip will be visible at the bow (half-round is more cosmetically pleasing) and is perpendicular to the water for much of that distance (half-round is more "hydrodynamic" than a flat bow seam). Then we switched to 3/4" x 1/8" flat bar for the rest of the keel (which is submerged out of view). The flat bar is cheaper, lighter, and easier to drill than the half-round, yet it protects just as well.
Although the 3/4" half-round can be hand-formed to follow the curve of the CS20's bow, the stainless steel has so much "spring back" that it could strain the fasteners more than they can bear. Likewise, if you try to bend it to the boat after the fastener holes are drilled, the half-round will have a tendency to "kink" at each fastener hole instead of bending smoothly.
We solved both problems by having our half-round custom rolled by Modern Welding of Georgia, located in downtown Augusta. We took them a cardboard template of the bow curve, and they took us out to the metal shop to get the half-round rolled into shape. The results were superb, and much better than we could've achieved by hand. The rolled half-round was almost an exact match to the bow, making it easy to fasten in place.
To fasten the stainless rub strip, we drilled oversized holes in the keel and embedded stainless steel nuts in semi-thickened epoxy. When the epoxy cured, we touched up the holes a bit with thickened epoxy, then sanded everything smooth and attached the rub strip. When tightening the fasteners, you must be very careful not to overtighten them or you risk pulling the embedded nuts right out of the hull. We learned this the hard way and had to re-embed two of the nuts, but in the end, it came out well.
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