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Author Topic: Sealing and strengthening a hull  (Read 1505 times)

Ron Couzens

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Sealing and strengthening a hull
« on: September 21, 2014, 09:17:05 PM »

I have never used tissue paper & dope to seal a hull.
Having read a number of articles on the subject I have found a number of options.
Could someone please point me in the right direction?
There is also mention of shrinking & non-shrinking dope, what are the advantages between one and the other?
Regards
Ron C
 
Just recently got back into building model boats having retired.
 
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Re: Sealing and strengthening a hull
« Reply #1 on: September 22, 2014, 11:28:34 AM »

 :-)Hello Ron,

I can offer an opinion based on some experience. :-))

Firstly, Dope resin was a good option years ago before the advent of more modern alternatives. One of the issues with using 'Dope' is just what the name implies. If you have ever used it, it is real heady stuff, and can make you feel quite strange & woosey [sic?] It is not something you want to use in an enclosed space, or, when there is no one else about. That is, in the shed & on your own. I have had experience of this resin when working on Spitfires & Hurricanes in the past, whilst covering the flying surfaces & the Hurricanes Fuselage. Yes, the real thing, and yet I am only 40 years old.

That was my early Career, but latterly, I have been involved in Design & Boat building full size for some time.

As a suggestion, I would use Epoxy PRIMER/SEALER (not the Adhesive Resin), with no Tissue or Glass (unless you have a serious issue with hull strength), in which case, better to apply Tissue or glass inside, if practical & really needed.. This stuff        http://www.mbfg.co.uk/epoxy-resins/eposeal-300-epoxy-primer.html  click this link or paste this into your internet browser, and have a read of its characteristics. The reasons are as follows;

1. You are building a scale model of a planing hull, with spray rails I presume, therefore thick adhesive resins (especially used with any Tissue or Glass mat) will most likely require a fair degree of post application sanding down, to make all smooth again. The danger here is that you could lose hull shape definition & alter the effectiveness of the spray rails. Spray rails need ultra sharp edges to be fully effective. Some might say that this is not important on a model, I say different <*<, it is even more important, having done real tank testing for real models for full size craft performance prediction, and analysed the results. You will also need as sharp an edge as you can achieve on the bottom corner of the Transom below the waterline.

2. Another is weight. All boats, full size and models, can easily end up over weight (models more so). For planing craft, keeping weight down during build is one of the most important factors. You can always add weight later if you need to ballast her down to her Design Water Line, and/or to achieve the correct Trim. In fact, this would be the desired outcome when you are ready to go for trials. A good starting point for the Longitudinal Centre of Gravity (LCG) is around 55% aft. That is to say, 55% of the vessel length as measured from the stem head  (or top point of the bow) towards the Stern. In any case it needs to be aft of Midships.

3. Epoxy has a low solvent content compared to most other resins, so it is a safer alternative for many reasons. It will soak well into the wood fibres, and it is one of the least affected by water, that is to say it is one of the most water proof. It is also readily available, if a little costly.

4. A couple of coats of Epoxy sealer will be easily applied, and quicker, compared to using a reinforcement as well.

If I were doing it, I would apply as many coats as it takes until it fills the wood grain completely, then sand gently down to make all smooth. Because you are likely to have developable surfaces which make up the hull shape on this model, as opposed to compound curvature, then sticking small pieces of sanding paper to small blocks of flat straight wood is an ideal way to go. Just cut the desired sizes, glue to the wood with a fine film of PVA, press them together in a vice until cured, hey presto, flat sanding blocks any size you could wish in just an hour or two. :-)

You may find that the first sealing coat raises the grain a little.  {:-{ Don't worry, just gently sand back until a smooth, matt finish is achieved all over, and then apply the next coat. I suggest 240 grit wet 'n' dry used with wet slightly soapy warm water. Then for final finishing, move to 320, then 400 and finally 800 in preparation for the paint.

Cost-I appreciate that this sealer stuff in not cheap.  :((   However, you will not use all of it for your boat, it has a long pot life, and therefore can be used for the next model, or around the house etc...may even be the same cost compared to Dope and Tissue combined, I don't know.

On a final note, do this all on a reasonably warm dry day, to prevent amine blush on the epoxy.

I am sorry if all this is a little long winded, or, if I am telling you things you all ready know.

Any questions, then I would be happy to help where I can. O0


Regards,
John
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