DRYING TECHNIQUES

Copyright ©1998 Steve Smith, All Rights Reserved.


How and why to dry a Ferro cement hull, a fibre glass hull, a plywood or balsa core deck, or almost anything else that is wet on the inside


It may happen that you have a boat deck or hull made of wood between two layers of glass-reinforced polyester resin (GRP). It may further be that construction or maintenance was deficient and water leaked into the wood core of the structure. Such leaks most commonly derive from no caulking or bedding compound, or a pilot hole drilled for a screw but the screw was forgotten and similar workmanship.

It may happen that you have a Ferro cement boat, and the paint does not stick on some areas of the hull. The pattern of paint failure may be little blisters in some region, or the paint may come off in sheets. Fillers may not stick to apparently dry concrete and the failure may be apparent in the yard a day later, or in the water a year later. The filler may turn to mush after water immersion, but that is invariably due to poor filler mixing.

It has been my observation (and that of Jim Campbell, a pioneer in the application of modern adhesives and coating to Ferro cement boats) over a period of thirty-five years, that epoxy adhesives and coatings can stick for many years (more that ten, with no definite failure point) to dry, fully cured concrete. It has further been observed that in every case where an organic coating known to stick to such cement does not stick in a particular case, that either the concrete was not sufficiently cured (e.g. 5 years at about 70° F or a silicone water-repellent had been applied) or this is the most common: there is some source of water that keeps the concrete wet.


The three most common sources of water in Ferro cement hulls are:

  1. a void in the concrete, usually in the thickest section above the keel
  2. leaking integral tanks
  3. a paint which is an effective moisture-diffusion barrier applied simply everywhere inside the hull.

The reason the latter creates a problem is that a Ferro cement boat, sitting in water will inevitably get some water on and in the concrete. That will diffuse through the concrete and can evaporate out of bare concrete or that which has a breathing coating. If all water evaporation is prevented, the concrete will eventually saturate with water and then external underwater paints fail.

Similarly, gel-coat blisters in GRP boat hulls are caused by water diffusing into the hull faster that it can evaporate off the inside surface, raising the concentration of water in the GRP laminate and thus promoting the chemical decomposition of polyester resin. The decomposition products of this reaction are water-soluble and promote the osmotic diffusion of more water into the laminate, create blisters. In order to stop and prevent such decomposition, a moisture diffusion barrier coating is applied to the outside of the GRP laminate hull.

A Ferro cement hull has different liabilities and a similar cure. Excessive salt water in the hull promotes the resting of the steel reinforcing. When steel rusts, it expands. Eventually the hull develops many cracks, chunks of concrete pop off, and by then it is too late. An intact moisture diffusion-barrier coating on the outside, and the available means for moisture to evaporate off the inside surface, and this failure mechanism is prevented. That has been my observation of the various boats done in various ways over 35 years, seeing what failed and what did not.

All these problems and solution have in common the need to dry something on the inside when water cannot easily or adequately evaporate off the outside surfaces. There is a technique for drying such regions, so that rot in a resin/wood/resin laminate can be dealt with, or coating failure addressed. I invented this technique about 1975. It consists of vacuum drying the affected laminate or cement structure from the inside. The principle involved is simply that water evaporates more rapidly at lower pressure. A vacuum pump of the particular can reduce the pressure one fifth of an atmosphere or less, causing water to evaporate much faster, and drawing it out of where you cannot get to it without dissecting the structure. In order to measure progress, the vacuum pump exhaust condensate is captured and measure every 6-12 hours and this then can be plotted. When the collected condensate graph falls from a higher to a lower level then the system is collecting only condensate from atmospheric moisture from air leaking into the hull or laminate. That means you are done.

A conventional protective coating system may be applied and usually everything works fine. There are, however, no guarantees. Your application is beyond our control, life contains hidden data, and the universe is not always fair.

Where more pumping speed is desired or there are many are leaks, such as a porous Ferro cement hull, a bigger unit may be required. The particular kind of vacuum pump we recommend is an Oiliness Rotary-Vane vacuum pump with at least 1/3 HP unit. When drying a Ferro cement hull, I would recommend you drill with a masonry drill a hole into the thickest part of the hull, above the keel, about every foot or two along its length. Glue a hose-barb-to-pipe-thread adapter into each hole. Use thick-wall clear vinyl tubing to tie them all to a manifold on the inlet to the vacuum pump. Leave any old paint on the hull, as it will only cut down the air load on the pump. The exhaust of the pump may be hot if it is pumping a significant air load. In this case you will lose the condensate in the hot air and you need that condensate so you can measure it. Try a coil of half inch copper tubing, perhaps fifteen feet in length on the pump exhaust. That should cool the exhaust air enough so you can collect it.

With a wood-cored deck you will need to drill holes every so often and glue in similar fittings. The spacing between fittings may be every two or three feet... what is important is that one fitting be in every region of damp wood. When a vacuum is applied to this kind of structure the top and bottom skins of the laminate are pulled toward each other with a force of more that ten pound on every square inch, equivalent to more that 1400 pound per square foot. This can seal off damp areas from the effective pumping action of the vacuum pump, preventing drying.

GRP (fibre glass) boat hulls that develop geo-coat blisters can be dried by a variation of this method, when in the gel cost is ground off (or at least, every blister opened and then a vacuum blanket put on the outside. This is a sheet of 6 mil polyethylene (clear so you can see if any interesting is happening) on top of a blanket (any thick porous material should do) taped up against a the hull with duct tape all around to seal. The vacuum pump penetrates the polyethylene (seal it with more duct tape) and becomes a manifold of smaller tubes, or preferably half-tubes or tubes with many small hole. The idea is to have a high-conductance gas (water vapour) path from all over the surface to be vacuum-dried so water vapour can easily get to the pump. The advantage of vacuum-drying a GRP hull is that otherwise one can pay for many months of being hauled out in an expensive yard with dehumidifiers in a tent below the boat in the summer. With a vacuum blanket you can actually dry the hull in the winter in the rain.

The drying process may take only a few days but may take much longer...every goat is different. You should budget two weeks for drying, and more time to set up.

Once the excess water has been removed from the structure, you can actually use those same fittings to impregnate the structure the Clear Penetrating Epoxy Sealer (CPES™). That glues together the slightly deteriorated wood fibre and leaves the entire porosity of the structure impregnated with a hydrophobic epoxy resin system, doing good things for the probably life expectancy. Do not use more than five pounds pressure to force CPES™ into a Ferro cement hull...one instance at tem pounds blew off the entire transom... Preferably, use only gravity feed from within a few feet of the deck. Put funnels in the tubing ends and pour CPES™ into the ones at bow and stern. Look for liquid showing up in the next ones. If it is milky let it drain until clear, then feed into those and thus progressively impregnate an entire fine network of voids. sometimes there is one huge void in a Ferro cement hull, and when the impregnation is done you will drain out quire bit when you drop the filling tubes to ground level...have bucket handy.

A GRP hull with get-coat blisters may easily be impregnated with CPES™ using a foam roller.

Having impregnated the structure, the solvents in that product must be removed in order to obtain the full benefit of that treatment, and that solvent removal must be done promptly, before the resin system cures. a vacuum pump may be used for the purpose, but there is the additional concern that the vapour, and condensate from the pump and the exhaust of that pump have flammable liquid and the vapour maybe explosive, as is the vapour of any flammable liquid. This sort of work is yet done by someone familiar with the safe use of plumbing and electrical equipment.

Once the impregnate solvents that dried any void may be filled with liquid or pat epoxy fillers as appropriate.