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MacandRita

Cobra fix up

14 posts in this topic

Hi everyone,

I have pulled my timber Cobra out of it's former life as a mobile storage shelf and now that I have a better work life balance, I am in the process of fixing up the issues and rejoining the local sailing club. 

The last time I did this I ended up running with just the high build as the paint job. The boat is I think is a Darryl Beattie built boat once called Tightrope. She is now known as SBNPJ (same boat new paint job).

Main issue is cracks in the outer layers of the ply. The inside of the boat was treated with epoxy so the inside layer of the ply is intact and fine. Only one crack has managed to get right through. I have chosen to use a router to gouge out half of the depth of the ply, then put down a layer of woven cloth and epoxy and fill the rest with epoxy and either sawdust or micro balloons. Most of the cracks are where the ply leaves the stringer. Is the cloth layer overkill or essential as it is a far easier process to just gouge out and fill.

I also have a couple of stripped screw holes that I will drill out and fill with bog and then redrill neat for the screw. 

I have sanded back the high build to remove the oxidisation that is present. I then intend to recoat with high build and then a tough final cat. I will be doing all the work myself and am fairly handy. Any hints that will stop the paint job becoming a dogs breakfast greatly appreciated.

I bought a compressor yesterday which came with the prescribed attachments but thinking I might need to invest in a putty gun for the high build. Will definitely get a water trap and dedicated spray painting hose.

I add photos soon.

Mac

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Tips:

Stripped screw holes......

Resin in a match (not the red phosphorus bit)......

Or

Wrap the new screw in Teflon tape, fill the hole with resin and insert the screw. The Teflon will srop the thread from becoming stuck in place by the resin, it also acts as an excellent water barrier ;)

 

DON'T use car bog on plywood, bog is designed to expand and contract at a similar rate to a cars steel panel, it will crack away from the plywood and subsquently leak..... it also leaves unsightly hairline marks in the paint once it starts to break away.

Get some chopped mat glass..... by gathering up a small clump of the strands and forming a paintbrush shaped clump you can then simply cut off short lengths (say 5 or 6 mm long "wiskers"),  theses"wiskers" can be added to a microballoon mix for additional strength

Microballoon mix should be a thick consistency. ......... much like peanut butter or even slightly thicker. "AEROSIL" is just as good as microballoons but around half the price...... try eBay for it 

Another good filler is to simply add sawdust (from a jigsaw or tennon saw so its in a small chip form rather than actual dust)...... again just add it to your resin and stir , keep adding the sawdust untill its like peanut butter in its consistency

By trial and error you'll find that in some cases you can almost match the plys color by selecting the right timber to dust...... good skill to have on varnished ply

;)

 

 

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Putty spraygun.......

Dont waste your time or money

You want a gravity feed spraygun, your local panel beater shop should be able to help you out here...... cost for a reasonable gravity fed gun is around $130, any more than that is just an extravagance you wont need.

 

Hi-build primer self-airiates once its applied, by simply adding thinners it magically becomes ordinary primer......  I use a general purpose primer from "Motorspray", unthinned it acts exactly the same as hi-build primer, then I just thin it down to act as a normal primer. ....... cost from my local AutoPro is $75 for a 4lit tin

:)

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Thanks Pirate, this is exactly the sort of information I am chasing. The compressor actually came with a gravity feed spray gun so I guess I'll give that a go first up. I have a sawdust collection system under my circular saw table so have plenty to work with. My bog is Bote cote epoxy and micro balloons or sawdust. I took some photos yesterday which I'll post tomorrow. Family duties won't let me get anything done today but will involve a trip to Cairns and paint shops. SWMBO and forward hand is still coming up with a colour scheme but I have to keep reminding her that Zebra stripes and fancy patterns are well above my skill level and remember SBNPJ was all sky blue high build. 

Thanks for the other handy hints as well. Plenty of food for thought.

Mac

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When I have screws in timber I always use epoxy glue to hold the screw and seal the timber - always easy to remove and glue back in.

If you use epoxy resin as a glue - add aerosil to thicken and stop it running as much.  To make a bog from epoxy resin, add aerosil and micro balloons (or Q cells).

This will make a bog that is compatible with the resin / timber and easy to sand, and paint.

Be careful when painting,  that the spray primer is compatible with the existing paintwork.

 

Look forward to seeing the boat on the water:)

Rick

 

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Morning Everyone,

This morning I downloaded the photos of the repairs. What I did was rake out the cracks I found with a pick. This convinced me that the cracks were only in the first layer of ply. On the larger areas I'll use the wider router bit so the repair will be part way over the stringer. These are routed to a depth of 2 layers af the 3 ply. the smaller cracks I'll use a smaller router tip and go 1and a half plys depth. The boat trailer has a floor so we can carry all the camping gear, sail bag etc. Yes under all that some where is a collapsable kitchen sink. The sailbox lid/punt then sits over it and it all gets held in place with a single tiedown. You can see the strip planking exercise with the nice curves done for 2 reasons. 1, to see what strip planking curves would be like and 2, to allow room for the dolphin striker on the cat. I still have to finish off the inside of the sail box lid/punt. It comes in handy for putting out the red claw pots and is rugged enough to handle one of those electric trolling motors if I got too lazy to row.

Thank you for all your advice and assistance. Does anyone have any thoughts about what to use for the top coat and any advise on how to get an even finish?

Mac

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Thanks Matthew,

I had read the first thread verbatum before starting my project and now thanks to you I have done the same with the second.

I took to the cracks on the deck with the router this afternoon and coated the exposed timber with epoxy when I finished. Plan to have the decks ready for final sanding by Wednesday so I can start on the walls.

Really enjoying myself.

 

Mac

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Evening All,

 

Gotta love the tropics. I got the whole deck done in a day. I started by pulling some woven glass cloth apart to have individual strands, bit like long chopped mat. I then half filled the cavities with epoxy and pressed a reasonable amount of strands into the epoxy until wetted out. This had all dried to tacky by lunch so I applied the bog during the afternoon. I also veed out some cracks in the centerboard which were in non critical areas and bogged them up as well. Pulled out the renovator which had been on the shelf gathering dust since I bought it 12 months ago. Fairly happy with what it can do. Not happy that the holding bolt picked up a thread the second time I undid it and I had to tap it to clear it out. So an initial sand tomorrow to get rid of any high spots then flip it all over to start on the hulls. Trying to do this all without undoing the beams. Plenty of height and good beams overhead and pulley blocks to burn at the moment so I think I will manage. The outside surface of the rear starboard side between the rear beam and the transom has the biggest collection of fine cracks. I know I could place a layer of glass on the inside but as I am thinking of routing off at least 1.5 layers of ply to remove the cracks, then I could cover this with glass, epoxy and peel ply so the finished glass would still be below the original surface level and then finish with fairing bog. The peel ply would help control the epoxy, fill the weave and reduce the amount of sanding before fairing. As I noted before the inside layer of ply was epoxied when built and is good. This area would have been the area of maximum torture during construction. This will be the last thing I attempt so I can build some more confidence that I'll proceed this way.

 

Mac

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Mac

Just be careful removing the ply - you may find it is only 3 ply - and you maybe better to sand off 1 layer and fibreglass.

Taking off 1.5 layers of ply in this area may lead to an unwanted out come - unless you fibreglass inside first.

Rick

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Tend to agree with Rick..... 

 

You can actually weaken the area by over doing it, I'd go to your local hardware shop and get a much finer bit for the router ..... I'd be looking for one thats a max of 10mm wide but with a pencil point, set its depth to 2mm and just take away enough material to get clean timber.

A vee shaped cut will be quite strong and use a lot less filler.... google "vee butt weld"  images. Yeah its steel related but its the strength issue your chasing and this is simply stealing from one trade and applying to another.

I use a router/trimmer most of the time, it fits in tighter places and is way easier to control, the bits are far cheaper too.

The only time I'd use a big router is on the beam beds, speaking of which, I know you're not that keen to pull them off but it may pay to do them while your doing the surgery waiting game. Just do one at a time and you'll find its a far less stressful thing than you think...... hate to count the number of times I've seen boats all fixed up and have issues straight out of the blocks with poor pointing issues or leaking hulls..... a ridged catamaran is a must and its 99% the fault of "not having a look at the time" that causes the issue & then it turns ugly because the new paint is going to get damaged......

Pull the beams, at the very minimum you get to reseal them and thats priceless in my opinion

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Evening Guys,

Having an easy day and used up my Christmas vouchers for Big W and Bunnings buying stuff for the project and shed in general. I sanded back the bog jobs and my 20 year old orbital sander final gave up the ghost. Pity this didn't happen before I went shopping. Got a chance to try out the sanding part of the Renovator. Worked ok but a lot slower. Then the brand new vacuum cleaner which I hook up to the Triton vacuum dust collection system, decided to play silly buggers so it will be going back. As soon as the  vacuum stopped sanding dust appeared on the work. Never realised before how effective it had been. Mixed some more bog this afternoon and faired all the repairs ready for final sanding. Will be flipping the boat tomorrow.

I worried that if I pull the beams off it will be another 2 months before we get on the water. I'll want to get everything vertical, straight, parallel, and sitting on firm beds. Wonder where I have heard this before. The platform is very rigid and I will gouge out and redo the goo around the beams with fresh neutral sealant. I will probably go so far as to remove, check and reseal the bolts

Pirate, I have a collection of bits for my router and I have the bit you refer to. I actually used it on a couple of smaller cracks and also to chamfer the edges of the larger ones. I like your suggestion and even though there will be a lot of grooves, fairing will be a lot easier. My router is fairly bulky but I find it controllable when I plunge and pull it toward me. It has handles on both sides and pulling towards me allows me a good view of where the bit is in relation to the crack and keep it where it should be. Mandatory PPE in place of course. Rub off from working in Agriculture. The 20 year old latex gloves are a bit hit and miss though.

Other things that work for me. I mix my epoxy in small plastic measuring jugs. Once mixed I empty into wax paper cup and turn the jug upside down on a piece of thick cardboard. 4 hrs later I can easily peel off the thin skin left in the jug and start measuring again. I use plastic knives with a cm cut off the end to make them square ended. The top edge of the knife is straight so allows you to scrape the walls of the mixing jug for uniform mixing and getting most of the mixed product out of the jug. It also doubles as a spreader for bog.. Don't bother cleaning it just let the bog set and then pull the knife out. Epoxy will hold but not stick to plastic. Set mixed epoxy will come away from wax paper cup but bog no so much, I bought the cups and knives as bbq packs and a set of 2 measuring jugs from the reject shop. I carn't see me needing to buy any more for some time.

Mac

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Evening All,

I have dug out and glassed all the visible cracks and bogged and faired half. I'll sand back and bog the rest tomorrow. Then fair and hopefully undercoat the hulls Tuesday. Then pulling up so we can go on vacation. A cruise around Tassie followed by a week in Melbourne. Get back 18th Feb and start a new job Monday 22nd. Dropped into the Tinaroo sailing club an caught up with our friend who now sails an A class cat that he has set up with homemade foils. There was also a foiling Moth having a ball. Hhhmmm. Foiling Cobra?

Mac

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Evening All,

Back from holiday, new job is great and back into the Cobra repair. Looking at the image you can see darker lines which indicate where I routed out the cracks. Started fairing to remove some of the low spots and get a smoother surface before painting. You can also see where I glassed the transoms previously. Need to do some fairing there as well. Bought the high build so expect to try out the spray gun soon.

 

Mac

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