Clear Coating Mahogany Hulls With Spar Varnish - RC Groups
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Jun 24, 2008, 11:42 AM
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P. Tritle's Avatar

Clear Coating Mahogany Hulls With Spar Varnish

Where it all started

After years of fooling around trying to find the best alternatives to finished fiberglassed mahogany hulls I think I've stumbled onto something. The material is not new - and I think that's good, because the worst luck I've had in finishing mahogany hulls is with "new" products.

After working with everything from epoxy resin to water base polyurenthatne varnishes to automotive clear coats, both catalysed and not, the results have ranged from pretty good, but very time consuming (epoxy resin) to downright terrible. Without going into great detail, I'll never use water base PU again --the results were disastrous to the point of downright embarrasing! -- and depending on the resin, the automotive clear coats have ranged from OK to disastrous as well in that the finish blushed to a hazy gray over time.

Back to Basics

So this time I thought I'd have a go at the old tried and true oil base spar varnish. Then to keep the sanding and polishing time to a minimum, as with the thinned epoxy resin method I figured spraying it would be the way to go. And by spraying 2 relatively light coats, the shrinkage won't be as difficult to deal with in sanding the surface between coats.

The Next Step

Now that the first coat is applied, I'll let it cure for two or 3 days and wet sand with 600 grit sand paper to smooth out the little flaws, then a second coat will be applied as described in the photos. From there, it will be down to polish. So stay tuned, I'll post up the final results when she's done.

Last edited by P. Tritle; Jun 24, 2008 at 11:50 AM.
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Jun 24, 2008, 11:51 AM
Mmm, tugs...
patmat2350's Avatar
I've always been too impatient for spar varnish, even though I suspect it could be the "best" stuff around... Still, I'm really interested to see how it works out!
Jun 24, 2008, 11:53 AM
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P. Tritle's Avatar
I didn't think to mention going in that the hull was first finished with 2 oz. fiberglass cloth and 3 coats of fill using West Systems resin. The resin was sanded down to 600 gritt to remove all the surface boo-boo's before the clear coat was applied.

Jun 24, 2008, 11:56 AM
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P. Tritle's Avatar
Pat, Me too. but I've had my kicked on more then one occation, and after the last colasal failure using WBPU I figured it was time to quit trying to "beat the system" and get back to basics. And at this point, it looks like the old will definitely "out shine" the new.

Jun 24, 2008, 12:21 PM
3 Blades to the Wind
Shaun Hendricks's Avatar
Okay, I have to ask...

If you are trying to find an alternative to a "finished fiberglass hull" then why put spar varnish on a fiberglass finished hull? How would you know the actual results were what they'd be say, versus, bare wood?

I would think two thin coats of spar varnish wouldn't be nearly enough on bare wood. I'd guess two is okay on an FG'd hull... but why not just use resin at that point? I love your FG jobs.

Possibly six to ten coats of fully sanded varnish might provide similar protection to a thin fiberglass job. It would probably be about the same thickness as well, just minus the glass structural support. Maybe I'm missing something about what is being accomplished here. I might have to exchange my brain for a better one as the current one is a little lost!
Jun 24, 2008, 12:45 PM
Mmm, tugs...
patmat2350's Avatar
1. Epoxy resin has no UV protection, while spar varnish does. A real issue for a real boat, not likely an issue for a model with its limited outside exposure.

2. I THINK the point of the second coat is just to provide an easier-to-polish coating... building up and leveling epoxy is a pain, while sprayable coatings are much easier to deal with.

That's why I like lacquer as a top coat... available in spray cans (as is varnish), really fast drying, and easy to polish.
Jun 24, 2008, 12:54 PM
Registered User
P. Tritle's Avatar
Shaun, I've done finishes like this without the f-glass and they just don't hold up. Where the problem begins is the varnish cracking and leaking small ammounts of water into the planking which will cause the planks to curl, then before you know it, the outside looks a mess and the inside starts to take on water as well.

I've also tried f-glassing with the old Parsons .6 oz. cloth and had the same problem, though it took a bit longer. So far, I've found 2 oz. cloth to be the best alternative for finishing at a reasonable weight. Now for display only, 8 - 10 coats of sanded varnish would give a terrific finish

Jun 24, 2008, 01:39 PM
Sea Dragon-Lover
Umi_Ryuzuki's Avatar
I have never had good luck with any Polyurethane finish.
At some point a pinhead air bubble will appear, and from there
it just gets bigger till the entire coat almost just begins peeling away.

Good information on the cloth weights and moisture penetration.
I think the lightest cloth I have used is 1.5 oz..
Jun 24, 2008, 06:08 PM
3 Blades to the Wind
Shaun Hendricks's Avatar
Okay, I think I get it now. So what we are talking about is FINISHING an FG job with a different kind of coating. Makes more sense to me now.

Wouldn't you want to avoid most varnishes as they tend to yellow in the sun or over time? I don't know if that one is susceptible. I did know they tend to crack as well and you have to revarnish, but again, I don't know if they've resolved that problem.

One of the advantages of polyurethane is that it didn't have those problems, however, you guys are right, it's a pain in the tush to apply correctly. I've only been able to get it to go on right with an airbrush in a perfect 'mix' and application rate. Others have told me the same thing, it's all about the mix. That's why some insist upon rattle can PU over bulk mixes. The rattle can stuff tends to be at the right mix if it's 'new', ie: within 6 months of manufacture date. Older stuff is garbage. I've never gotten old stuff to work right. I only use it for protectant. PU paint is generally good though as the pigment is in the PU and if you can keep the bubbles out, it stays pretty okay.

In the end, both varnish and PU are fairly soft finishes. I like enamels for their hardness and below that, epoxies. I'm not sure if there is a best solution for going on after fiberglass but I'm glad to see someone trying it out. Keep it up Pat, and good luck! I've read all these threads and applaud your efforts.
Jun 25, 2008, 09:39 AM
Registered User
P. Tritle's Avatar
Shaun, That's right, the clear coat is used to eliminate the many hours of sanding and polishing the resin finish the high shine that can be easily sprayed instead.

The varnish is an amber color, but that actually works to your advantage when finishing a mahogany hull since the amber color actually compliments the wood finish.

I haven't worked with PU color coats, but I have used WBPU varnishes. In their place they work great, as in applying silk span over balsa. I've finished several boats that way with excellant results. However, as a finish coat the results were poor. The WBPU doesn't build up well, and after 5 or 6 coats the gloss goes to a satin sheen that won't polish out.

Jun 25, 2008, 11:42 AM
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Ed Crowell's Avatar

What is the ratio of laquer thinner to spar varnish that you used?

Jun 25, 2008, 12:22 PM
Registered User
linerfanatic's Avatar

Spar Varish broke my heart....

Boy was I glad to see a discussion on this topic. Last night at 10:00 pm, I took a sander to the deck of my 1949 Dumas Runabout and removed the 3 coats of Spar Varnish I had so carefully applied. The reason -the once crisp white deck caulking was now dull brown. It just looked awful.

I was heartbroken. I put so much work into keeping the styrene strips clean and free of stain - well - it was pretty discouraging. I was about to pick out a new urethane today at the store when I read Umi's posting that urethane was hard to work with. I really suspect he is spot on about that.

Now that I have sanded down through to the epoxy layer, the strips were back to bright white, so I can at least confirm that the varnish was the culprit. But man, the sides planks are drop-dead gorgeous with this varnish. I am tempted to leave those as they are.

I had used only Spar Urethane on my Dumas Cobra and it held up very well for many years. It really is a durable finish. Naturally, moisture intrusion forced this issue and stripped it down to the wood, added fiberglass/epoxy, re-varnished, and re-polished. That worked well as the "caulking" does NOT appear dull or brown on that model. So it is either the brand of Varnish I chose or this particular batch.

I would guess that I could leave the varnish on the sides and spray lacquer on the deck as the two shall never meet? I am only wondering if the deck will have a different appearance in color from the sides.

There are products that call themselves "Spar Varnish" and those who refer to the contents as "Spar Urethane". I don't know the difference and wonder if the "Urethane" Product is what I may have used on the Cobra. The product I just sanded off is "Spar Varnish"
Jun 25, 2008, 12:52 PM
3 Blades to the Wind
Shaun Hendricks's Avatar
LOL! I'll field this first....

UMI is a 'she', her avatar is a clue there. I know, most anime fans are guys, but plenty are girls! Most of us around her kind of worship her work, she makes models for a living too- lucky girl!

I'm not so sure the modern varnishes don't resist yellowing, but I'm glad you posted this up linerfanatic. Pat will have to let us know how it holds up over time I guess!
Jun 25, 2008, 02:49 PM
Registered User
linerfanatic's Avatar

Correction happily aknowledged.

Good heavens. To commit such a social error on my first posting! The ladies are more then welcome to join in on the fun. I will attempt to redeem my crime by posting some photos of my build.

Back to the varnish. Can I spray the laquear over my newly sanded and cleaned deck? Or will I need to get the deck sanded and cleaned to the point where no varnish molecules remain?
Jun 25, 2008, 02:57 PM
Tinkerer in Training
RGinCanada's Avatar

I think it is important to note that Pat fibreglassed the deck first, sealing the wood and caulking lines. The varnish is being applied as a top coat over the West System epoxy. If you apply varnish to your deck without this step, I suspect you are heading for another broken heart!

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