I’m always looking for the best coating for the canoes and kayaks I build and paddle. Wether it be a clear varnish or an opaque paint, whatever I use it has to meet two basic requirements. It has to protect the epoxy matrix from UV and abrasion damage. Epoxy will break down over time when exposed to the UV rays in sunlight so the coating must have a good amount of UV protection. Every boat will get dragged up onto beaches and hit rocks. That’s a fact. It’ll get wear from entry and egress. So it needs to rugged. Some other desirable attributes: It had to survive periodic immersions. It needs to be attractive, have a deep shine that lasts, and be easy to maintain. Lastly it also has to be cost effective to apply and polish.
Since January I’ve been running a little test of different coatings designed for marine/exterior uses (OK, UV and immersions â€“ check). So the test would be for gloss but more importantly for abrasion and application. I tested six (6) clear and six (6) opaque coatings. The clears included a one and two-component varnishes, a three-component automotive urethane urethanes a waterborne poly, polyester surfboard resin (similar to a gel-coat) and even just plain epoxy. The opaque finishes included one and two-component urethanes, a complete automotive finish (from primer though clear) and pair of graphite coatings.
It’s not very scientific but the results I saw enforce my decision to use two-component urethanes (ie. the automotive clear coat I used on the â€œNereidaâ€ and Interlux Perfection paint and varnish) for my boats and potentially added an very exciting new one to my stock. Click on each image for larger versions of each.
The whole idea to even run this test started last summer when I was having problems with the waterborne finishes. I knew the two-component coatings were supposed to be better but by how much?
Then this winter I found out about product I’ll call â€œWhite Graphiteâ€. I’m not giving out the exact name for commercial reasons (for private builds contact me off-line). It was suppose to behave very similar to the black West System 423 Graphite Powder. West 423 is fine, black powder, similar to toner. When added to epoxy it makes a very rugged surface with low friction. It’s often used for bearing surfaces and for the bottoms of expedition canoes. It polishes to a nice sheen and just slide over rocks with barely a scratch. Being epoxy based it is also able when timed correctly, to chemically bond with the underlying epoxy / fiberglass matrix. Neat stuff. The only problem is that it’s black as ebony. Then I heard about this â€œWhite Graphiteâ€ and thought â€œoh, I have to try this stuff out…â€ It’s base color is white. White hulls are very popular but it can be tinted giving the potential for a wide range of colors. If it worked it would make an amazing finish for at least a hull, maybe an entire boat. Luckily I found a local customer who was willing to use his boat as the test-bed for this new coating. I’ll be building his boat (another Njord) in the upcoming weeks. Until then I wanted to see how it was to apply, polish and how it would perform. Time for a head-to-head test with the other coatings.
At first I thought about making up some fiberglass-laminated cedar panels and coating them. I was building the â€œNereidaâ€ Njord at the time and didn’t have THAT much time. So I settled on laminating a layer of 4-oz fiberglass onto some 3/4â€ thick sanded ACX exterior fir plywood. All I really needed was a firm surface. I put the fiberglass on to simulate the outer skin. The coatings would need to bond to a final layer of epoxy anyway. Although the whole idea of the test was to test the coatings, not the fiberglass or the core, I did use two different types of fiberglass. I used regular E-glass for the top half of the panel and the stronger (by 30%) S-2 glass for the bottom. Would the S-2 glass be any better? Either way the glass will quickly allow me to see how deep the scratches go. Fiberglass turns white when scratched.
I sanded the ACX to P120 and laminated the fiberglass dry on wet using some left-over US Composites Medium epoxy. Each panel received two more fill coats of the epoxy and were then sanded even with P120. A final gloss coat was then rolled on. All the coatings would bond to this last gloss coat after being sanded to whatever grit was required by the coating manufacturer. I do a similar schedule with the boats.
After the panels had cured for a few weeks they got coated with 12 different coatings: six clear and six solid. A complete summary of dry film thicknesses and amount of time required for application is at the end. Here are the candidates (ie. stuff I had in stock):
- Plain old epoxy: Why not? Two coats of MAS resin with slow hardener. No UV protection, though.
- Polyester: Silmar 249 Surfboard Resin: I had some laying around from a previous project/experiment. It would be similar to the gel-coat found on commercial boats. One thick coat is usually applied. The stuff was old and never kicked so I ended up using a scrap I had made a while ago.
- System 3 WR-LPU: The now infamous waterborne poly that drove me nuts last year. I had some left over. Four coats applied with a foam brush.
- Epifane’s Wood Finish Gloss: It’s a traditional solvent-based marine varnish and gives a nice, amber hue to the wood. Pretty stuff but it takes forever to dry. Three coats.
- Interlux Perfection Varnish: A rugged two-component varnish that is now the staple of the shop. Hard to beat it. Two coats.
- DuPont ChromaPremier 72200S automotive urethane: I has Pete spray four coats of the stuff with the flex agent when he was clear coating the â€œNereidaâ€ Njord. This stuff is so pretty when sprayed… How would it hold up though?
- System 3 WR-LPU â€“ Orcas White: The now infamous waterborne poly that drove me nuts last year. Again I had some left over. Four coats applied with a foam brush.
- “White Graphite”: I made two sets of panels. One with two coats and one would three. Would the extra coat make any difference?
- Epifane’s Monourethane â€“ Light Oyster: A single component marine topsides paint that is very well regarded. Wonderful gloss and easy to apply but it takes 24 hours for each coat to cure. Three coats.
- Complete DuPont Automotive Refinishing System â€“ Red: Pete was repairing a Subaru Outback at the time the â€œNereidaâ€ was being cleared. I had him spray one of my panels with the same paint as the Subaru. There are two different primers, one color coat and one clear.
- Interlux Perfection â€“ Jet Black: A rugged two-component urethane that is now the staple of the shop. Hard to beat it. Two coats (I usually do three on my hulls though).
- MAS Epoxy with West 423 Graphite: I knew the stuff was tough. I mixed it in to the epoxy per the West instructions at a ratio of 10% by volume. Some people also add aerosil (fumed silica) for even greater abrasion resistance. I left it out.
To make the everything consistent I wet-sanded each panel to 1000 grit and then quickly polished them with automotive rubbing compounds. I polish every boat before it leaves so I wanted to see how easy the coatings were to polish. The two automotive finishes had the highest gloss and were the easiest to polish. Following closely was the two Perfections and the polyester resin. The others were mid-pack with the WR-LPU’s bringing up the rear. The solid paints were quite easy to burn through. I would apply and additional coat for polishing. The clear LPU turned hazy and the white took forever to polish. The graphites (both white and black) polished quite quickly to a very nice gloss. One strange thing was the white graphite had pores that filled with rubbing compound. I used the same bonnet for all samples. For polishing white I would use one bonnet exclusively. Still I was pleasantly surprised how glossy the graphites were. Very pretty with a silky feel. Nice.
I settled on a panel size of about 6â€ x 6â€ for some good point loading. I have a piece of railroad rail cut-off in the shop that weighed around 32 pounds. That would provide the downward pressure. I built a quick sled out of plywood to mount the panels and rail and to attach a rope for pulling. The downward pressure averages out to 0.89 PSI or 128 PSF. It doesn’t come close to the loading experienced when you hit a sharp rock but it should simulate a LOT of consistent wear like years of landing a loaded kayak on a gravel beach. What’s the exact number for that? I haven’t got a clue… I wouldn’t want to even guess. The test will at least give a consistent pressure from one panel to another.
Before I dragged each panel I weighed them on my gram scale. I should have done it before coating but didn’t think of it. I could have estimated the weight of each coating. Oh well… Like I said I mounted each panel to the bottom with some double-sided tape and then dragged each panel from my grass lawn across my crushed-gravel driveway and back to the lawn. It’s a distance of about 30 feet total. I pulled each one at a fairly constant walking pace (about 4 knots). I then weighed each panel after to see how much of the coating was abraded.
I then took each panel and rubbed them hard against a corner of the steel rail to simulate hitting a sharp rock. The only panels which did not scratch through the fiberglass were the graphites. They took the abuse and kept on smiling. All the other penetrated. Next best was the two-components and the clear DuPont 72200S. The worse were the colored automotive system and the polyester. The automotive system had a thin, cheap clear coat which provided limited protection. Coating it with the DuPont 72200s would greatly improve it’s results and would be a stunning finish. I think the polyester was again too thin to be effective.
For opaque colored finishes the best was the graphites. Both the hBN and West 423 passed with flying colors. Very similar performance. They both have some ablative properties. I had to drag it 5x the distance to get similar visual wear patterns. After one pass they had loss about 0.25% of their weight and 0.5% after five passes. Scratches take off a small amount of material instead of just cutting through it. So it wears very evenly but is very resilient: ie it’s hard to scratch through to the fiberglass.
Next best was the Perfection and other 2-component paints which is a little surprising since they are the thinnest in terms of dry film thickness (DFT). The singles and the waterborne were way too soft. They scratched through to the fiberglass quite easily. The complete auto-body finish looked amazing after being polished. With only one layer of a cheap clear though it didn’t hold up well at all. The scratches cut through the thin clear and then straight through the softer color and primers. To perform adequately it would need more coats of a different clear.
The clears performed similarly. DuPont auto 72200S, then Perfection, then the singles. The 72200s was incredible. Very rugged as well. Easily the equal to Perfection. The polyester was mid pack. The varnish did ok for what it is… marine varnish. You expect to recoat it every year or two. At the bottom was the waterborne and the traditional varnish. The waterborne was soft and just turned cloudy when scratched. What was neat was the straight epoxy performed quite well. It polishes up nicely and was pretty rugged. Too bad it has no UV protection.
You may be wondering about the higher-strength S-s glass… Did it have any impact on the test? Short answer: not much, maybe. From what I could see on individual panels scratches that penetrated the glass were slightly shallower. It also could have been how the panels were rigged in the sled. S-2 glass is 30% stronger than regular E-glass so you can you less glass for the same overall strength. Or on the flip side, use the same amount and get a stiffer composite. It’s only really needed if weight is a concern, like for racing.
The test reinforced my decision to abandon the waterborne WR-LPU wholesale and switch to two-component solvent-based urethanes like Interlux Perfection. There was no comparison. The single-component finishes (Mono-Urethane and traditional varnishes) remind me of years gone by where owners of wooden boats would re-paint their boats every few years. Today’s modern two and three component systems just outperform them handily. They are more expensive up front but the difference in performance was staggering.
The DuPont 72200s clear coat with the flex additive performed extremely well. I’m glad I went with it for the â€œNereidaâ€ Njord. It’s a bit pricey but the finish speaks for itself. The complete automotive system would be outstanding if combined with the 72200S, especially on a deck.
Lastly I was incredibly impressed with the graphites, both white and black. It’s no wonder why a number of people with expedition canoes coat the bottoms with graphite. I’m thinking of re-coating the hull of my personal Guillemot with white graphite this summer for use as another test-bed. If the results of the full-scale test this summer on the other Njord are favorable I’m going to recommend all painted boats have a graphite-coated hull. Stay tuned…
Urethane Safety Warning!
Two and three-component urethanes should only be sprayed using forced air systems and be done by professionals.Â Spraying them is definitely not for your home builder.Â Highly toxic isocyanates that are produced during cure. The side effects can be very serious and are cumulative. Small exposures add up over time and are not reversible. Isocyanate fumes readily penetrate exposed skin and eye membranes and are not generally detectable to the human nose at levels that are toxic. In fact the DuPont material can only be purchased by commercial spray shops that have the equipment and training to use it correctly. I have my boat professionally sprayed by a guy who has been spraying custom cars for 30 years.Â Pete has the proper equipment, training and experience to spray them.Â I am not set up to spray them.Â Doing so as currently equipped would endanger the environment, my health, the health of my family and would probably be illegal in NH.
Notice how I said “spraying.”Â From my own research, training and personal use, the Perfections (Perfections only!) are fine to use if they are applied by rolling and tipping or by brushing.Â For the abrasion tests I brushed both the Jet Black Perfection and the Perfection Varnish.Â Since you’re not atomizing it, only a very small amount of material gets put into the air.Â Most of what gets put into the air are the reducers and not the isocyanates.Â Those have already bonded with the resins.Â You get into serious trouble when you atomize a liquid, especially one with isocyanates in them.Â Everything is then atomzed.Â You still need to wear some PPE like a charcoal filter mask because the reducers are pretty nasty and wear gloves to keep it off your skin.Â I also recommend doing it in a place with plenty of ventilation.Â Because it cures so quickly you can even do it outside. Any dust nibs or insects can be polished out.
I didn’t mean to make light of the dangers involved.