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Old Jun 19, 2012, 04:20 AM
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WOODY: Problem filling gaps between planks during finishing

I have been busy building my first planked wooden runabout and I am in the finishing stage right now. To quickly lay out the curcumstances and the problem, let me describe how its put together. The hard-chined hull was built upside down on the plans, sheeted with 1mm beech ply, then the bottom skins received 2 layers of glass using west systems resin, which came out great. The sides are planked with 2x7mm mahogany as is the transom.

The deck was built separately on the plans from a 3mm ply frame, planked first with 2mm balsa, shaped and sanded, before being planked with alternating strips of 7mm wide mahogany and 2mm wide obechi.

I planned to apply glass over the deck and sides, squeegy-ing off the excess resin, after which another coat of resin to fill the weave and level the surface. This was to be sanded thoroughly in preparation for 5-6 coats of spar varnish. I know this was the best method but I got nervous about trapping bubbles under the glass and decided to skip the glass cloth, sealing the wood with 2 well sanded coats of resin. Still can see some small gaps in the finish between planks, but so far, so good. Next I wiped the hull down with silicon remover/degreaser, and applied an admittedly heavy coat of catalyzed clearcoat, which blistered just as it was curing, gaps still not filled in. The next day I used a random orbital sander to sand it all off as best I could, taking it back down to the resin-soaked wood.

Once it was all looking even I applied a thin coat of thinned out catalyzed clearcoat, at which point the gaps are still present, but as I cant exactly primer this thing I expect subsequent coats will fill them in and level it all out. Now I have started with spar varnish, it looks fantastic except for those damned cracks. These things are narrow im telling you just 0.25mm tops. After I brushed out the first coat nice and smooth I could see the cracks have absorbed the varnish from the top so went back with a very fine brush and applied an extra few drops of varnish directly to the cracks. This all flowed out and looked like the problem is at last solved. Then this morning I went in the shop to take a look and those damn cracks are still there, the varnish has some how been repelled out of the cracks and left a high spot on either side which now must be sanded out.

After thinking about it I am convinced the resin in these cracks wouldn't have been properly sanded and probably still contain amine blush, which I read on the internet will spoil a paint job. My question is what to do to correct this problem, at this point. My idea is to get a small fine brass bristly brush and dry scrub the cracks to try and remove the top skin of the resin, then repeat with the brushed dipped in silicon remover/degreaser. Then a thorough blast of air and leave to dry and off gas overnight. Tomorrow apply new varnish only to the cracks with a small brush and leave to dry. If that doesn't solve it my last resort is to strip it all down to bare wood again with 80 grit, scrub the cracks, and start over with a layer of glass cloth.

Anyone have any other suggestions? I'm losing steam on this project and I would hate to shelve it at such a late stage. I had hoped to have her on the river this weekend.

best regards,
Jayjay
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Old Jun 19, 2012, 06:22 AM
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Bradford West Yorkshire, UK
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Either use surfacing tissue (glass mat) or lightweight woven glass cloth. You could probably have got away with using a body filler as you state "Paint job" which leads one to believe that's going to be a colour coat.

Even if you're going to clear coat to show the wood grain, tissue or cloth will disappear when resin coated.

Regards Ian.
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Old Jun 19, 2012, 07:35 AM
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so your recommendation is to strip it back to bare wood and start with a layer of glass cloth, and not mess with trying to fix the problem? That will be my last resort. I picked up a brass brush at lunchtime and did a bit of work on one of the cracks, then used a needle to clear out the debris. I'll rub it out with thinner and dab some more varnish on this spot to test, if that doesn't get repelled back out of the crack ill do the same to all the others. If it doesn't work I'm going to fire up the sander and strip it back down. I have 2mm planking to work with so I'm pretty sure I can still strip it back without causing any problems.
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Old Jun 19, 2012, 09:23 AM
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Will the varnish ever harden enough that I can wet sand and polish it or will it need a final spray coat of thinned varnish to make it shine?
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Old Jun 20, 2012, 03:32 AM
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You are using an epoxy resin?

Regards Ian.

Edit, have a look at this post :- http://www.rcgroups.com/forums/showthread.php?t=1675169
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Old Jun 20, 2012, 04:47 AM
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Whether I've been lucky or what, don't know but my own limited experience with epoxy skinning, I've never seen "Amine Blush". The surface has hardened up and after a suitable time, responded well to a wet application of wet and dry abrasive papers.

Regards Ian.
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Old Jun 20, 2012, 08:19 AM
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I used west systems epoxy resin. I dabbed varnish on the cracks last night and put it in the compressor room which is always very hot and dry. This morning its looking better, I think it needs a few more coats brushed on before I wet sand. Is it critical to scuff the varnish before re-coating? if that's the case I might just wet sand it lightly to knock off the high spots around the cracks before scuffing the whole thing.
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