wooden frame sealing, or not?

hi,

I'm building a small cat with plywood hulls over a wooden frame. Plywood is exterior grade okume and frame is spruce ( I know not the best wood for this, but it is what I can find locally, and at least it is light and easy to work with... ).

I will cover the outside with epoxy fiberglass (probably a single layer of cloth, heavier on the bottom and lighter on the side). I have doubts on how to handle the inside:
* option 1: do nothing, leave the bare wood, trust than the outside skin will keep it dry, and if ever gets occasionally wet it should just dry (as long as this is an occasional event)
* option 2: seal everything inside with epoxy, or epoxy AND fiberglass (I heard that plain epoxy can crack over time...)
* option 3: paint the inside with something to protect the wood from eventual water damage ... enameal? some rot protection product?

These are cat hulls, to they will not get water inside unless there is a spill/damage in the outside skin, it is my first build so I'm keeping things simple, I do not expect this to last forever, in a few years I'll probably want to build something better ^_^ .

I'm leaning towards #1 but I'll appreciate every opinion I can get ^_^ thanks!

 

This sounds like an interesting project - how big is your cat, and will it be power or sail?
Will the hulls be sealed shut totally, or will they have inspection hatches in the top for access?
Do you have any photos that you can post to show your progress so far?

I would be inclined to seal the plywood on the inside with a couple of coats of epoxy resin.
If you have access inside the hulls for inspection, don't paint over the epoxy afterwards - if you paint it, you won't then be able to see any areas of rot that might start later.

 

I’d go with the second option; especially if you can slather in epoxy after the framing from above. Just don’t pour too deep.

Typical coverage of 2-6 oz per sqyd will end up at say 2 sqyd per hull length or say 20 square yards and an extra gallon or two of epoxy.

 

The usual and also my recommendation is three coats of epoxy and if the interior sees sunlight an additional coat of paint or laquer to protect the epoxy from UV.

Normal outdoor wood paint works well with woods that have higher rot resistance, but both spruce and okoume would need some rot preservative before paint.

Do nothing isn't a good option, the wood absorbs moisture from the air and small cat hulls don't have good ventilation, so rot will soon follow. The reason we paint (or oil) wood, is to slow down the woods natural moisture absorbtion rate. From the commonly available coatings epoxy works best.

 

My experience:

I coated my 2x6 Douglas Fir frames with Cuprinol ( Homepage | Cuprinol https://www.cuprinol.co.uk/en ).
The frames support 3/4" plywood with ample polyester fiberglassing on the outside.
That was 26 years ago. The bilge has remained dry and is well ventilated.

Good luck!

 

thank you all for the replies!

quite small, 5mt-ish, it will be a platform for a small shanty boat thing XD
the hulls will have hatches for inspection, and will probably hold some techincal stuff like batteries and water tanks
at the moment I have a bunch of frames and stringers, once I have an assembled frame I'll try to start a proper worklog somewhere ^_^