The question that has been asked on this Topic i.e. whether or not to paint the inside of a wooden hull – give it some form of sealant or protection; is one of those questions which has no real definite answer.
This is because the answer will come from the person’s own knowledge and experience.
However, to make it easier to make a decision – rather than falling into the pitfalls of saying ‘well I paint the inside of my hull and it’s so good and lasted so many years etc., - or – I don’t paint the inside of my hull – and its lasted for years and years’….
Let us have a look at some basic facts.
Let us have a look at our model – how much time does it spend on the water? On average I would suspect that a model would spend at least 2 hours per week on the water – Sunday morning sail; and the rest of the week – normally being kept somewhere dry and reasonably damp free – in a stabilized environment. In other words, it’s not in central heating one minute and then outside in the next.
Really the water doesn’t have much time to penetrate into the fibres of the timber. That brings us to the next question we have to satisfy ourselves with ….
What timber is the hull made from? There are a lot of hulls made with balsa wood – in the early 1950’s - 1960’s which are still around today. This is because: -
Balsa wood (as well as absorbing water quickly) dries out quickly as well. With this though when the balsa wood is wet – it does swell and distort a lot more than (shall we say) mahogany or Obechi. For comparison if we have balsa wood which rapidly swells up when wet/damp we will have timbers like teak, mahogany and oak which swell but not as much and in between we have the likes of pine/Obechi which are roughly in the middle of the table of how much timber does swell up at a given intake of water.
So, we will take our Balsa wood hull; if we painted it with emulsion paints; this is one form of barrier to water which has been around for generations and, although the paint isn’t 100% waterproof (unless it’s a 2-part epoxy paint) – over a given time the water will penetrate the paint barrier and begin to enter the timber. This timeline isn’t 2 hours though, like the time the hull is actually in the water. The hull would have to be permanently in the water for a couple of months, before we saw signs of water penetration.
One of the other good characteristics of using paints directly onto soft woods; is that the paint will actually expand and contract along with the wood. So, on a warm day when the wood expands – so will the paint – and it will be less likely for the paint to peel off. Unless of course there is some defect between the paint and the wood.
Now, if we have the outside of the hull sealed with a paint barrier, but do not seal the inside of the hull, although – unless it is an open boat – there is little chance of water actually entering the inside of the hull unless the hull is actually breached. If this is the case, apart from our model disappearing out of sight in a downward motion, this is going to allow water on the inside to the unprotected timbers. This is where our trouble may begin. As the planks begin to swell they try and force themselves off fixed positions – in other words where we have secured them to the frames and keels. Some types of glue may accept this expansion and stretch with the plank, but, some may not and you may get failure in the spot. Don’t forget the majority of the planks in the hull are glued to one another by the edge, so this is going to restrict the movement of the plank and this will cause cracking or splitting. This scenario I am speaking about is going to the extreme where the hull has been submerged in water.
On average then the hull which is made from soft planks and unprotected should survive a long length of time, as long as the exterior is kept in good condition and frequently maintained.
Now, the average modeller is like me – I PRESUME – and yes Dicky we do sometimes sail our models!
When we lift them out of the water, we disconnect the battery and wipe dry the bottom of the hull to stop it dripping all over the car, whilst transporting it back home. When at home, we remove the batteries from the model to recharge them for the next sail – and we place the model promptly back on the shelf and forget about it.
How many times do we check the inside of the model, behind each bulk head looking for water which has crept in via a leaky prop shaft/rudder post?
The only time we actually do this is when we see pools of water in the bottom of our model boat.
This is when – a hull which hasn’t been sealed on the inside, suffers the most.
The other thing to look at is for those of us, like me, who use a polyester resin to seal the outside of a hull as well as giving a good finish to the hull. This polyester resin BE WARNED is not 100% water proof. Over a given time, if constantly submerged, will allow water to penetrate.
Also, this polyester resin requires a top coat of either some form of paint; which in top has some form of varnish – the other thing about polyester is, that it is prone to UV rays/sunlight rays which do deteriorate it over time.
One of the disadvantages of using polyester resin and not sealing the inside is, polyester resin expands at a slower rate than wood. Therefore if the inside of the hull becomes wet and it expands it will have a tendency to either split away from the polyester resin skin or even crack it. This is why it is always best, if using polyester resin on the outside, to seal the inside.
So, you are in actual fact encapsulating the timbers and sealing them for life.
Although we paint the outside of a polyester coated hull; the only time to either varnish over the top of polyester resin on the inside, is, if we are building an open launch. We could use the more expensive epoxies and you will find these are stronger and more durable than polyester resins. They do have a greater resistance to water absorption especially those which have been manufactured for the marine market – but, these are a lot more expensive. It is one of those decisions where – does the hull and the time it spends on the water – warrant the expense of a more expensive epoxy.
Last but not least, some hulls are manufactured with modelling quality plywood. Now this does have different characteristics to normal wood – due to the fact that the plywood is built up of thin veneer layers which are laminated together and the glue which is used to laminate them, do 2 things. It controls the expansion of the plywood when it is wet and also, it also forms a barrier – that is if the material used for bonding is of a marine grade.
What would happen, is, that the water would penetrate the first veneer of ply, but, would be unable to penetrate through the glue bond, unless of course there are any defects in the bond.
A hull made from plywood – I would still personally seal the inside as well as the outside.
One last thing about sealing the inside; when you do this, whichever materials you use – either resin or paint – it will sink into all the little nooks and crannies and fill & seal them thus preventing any water/dampness accumulating in this area and started rot.
Hope this has instilled some food for thought.
Aye
John e
bluebird