> I am repainting my boat and forgot a couple of things that maybe I use to
> know. How long should you wait until you sand, after applying paint? How
> long does it take a one part paint (interlux bright sides) to really
> really dry hard and completely (boat is stored outside in high 60 degree
> weather during day, upper 50s at night, and variable humidity but mostly
> dry)? How much weight would you guess a layer of paint would add (I have
> sanded the older stuff, but not down to the wood, so I will be adding some
> weight to boat)?

> Thanks, Dave Walker

Most of this information should be on the side of the can -- it tends to be in really fine print -- and you should consult that, or call the manufacturer, for the final word. If you can't read the info on a pint can, and don't have a magnifying glass handy, find a gallon can of the same stuff. Some places use bigger type on the labels on the bigger cans, and awful small type on the small cans.

Paints, varnishes, and other finishes can 'dry' in a variety of ways, but the basic ways are oxidation, polymerization and evaporation of solvents.

For example: In a lacquer or shellac, the plastic material is dissolved in a solvent, and a pigment or other materials may be added. When the lacquer is spread out in a thin film there is a lot of surface area exposed, and the solvent can evaporate rapidly, leaving behind the plastic base and the pigments trapped in it. There are several solvents on the market that may be used. These chemicals evaporate at different rates, and cost different amounts. manufacturers may mix these to achieve low price, or rapid drying time, or for other reasons.

U.S. Govt. guidelines on the evaporation of Volatile Organic Compounds (VOCs) can also play a role in the choice of solvents, and amounts in a given can of paint. Freon is probably the most famous VOC (it doubled as a solvent and a propellant in spray paints) that has disappeared as a result of its negative environmental properties. In some cases solvents are used which have known health risks. So, read all warnings and use proper safety equipment.

On many cans of paint the percentages of pigment, binders and solvents are shown. They figure these percentages by weight. So, if your paint is 75% solvent, then after it dries you will have only 25% of the original weight left. If a quart of paint weighs in at 2 pounds, then when it has dried, the paint film will have added 25% of that, or 1/2 a pound to your boat. On the other hand, if you sand off all the paint currently on your boat, you will lighten it by the same 1/2 pound, and if you leave a bit on, then the amount is pretty trivial in regards to weight.

Paints which are epoxy based `dry' in the same way that epoxy resin (like we usually use for fiberglassing and sealing seams) does. That is, it polymerizes, or forms complicated, long, molecular chains as a result of the chemical action of the two mixed components, the resin and hardener. There may be small amounts of solvents in epoxy resins, but these are trivial in weight. Basically, since little or nothing evaporates from the liquid epoxy, if you mix a 2 pound batch and apply it, then it will still weigh 2 pounds after it has `dried'.

Some oils will thicken by a polymerization process that involves reacting with oxygen in the atmosphere. They oxidize. In theory this should make them actually get a bit heavier, as they are picking up additional mass, over and above what you apply, but in practice paints made with these oils usually have some thinners in them which evaporate. I won't try to estimate the change in weight with these types of materials.

How long you have to wait after applying a finish and sanding it varies with the material. Same goes for the length of time for something to `completely dry' with epoxy and oxidizing oils, the polymerization process can go on for weeks.

To give an analogy. How long do you have to drive a car before it loses that new car smell? Part of the aroma is from trapped solvents that are being released as something continues to cure. The same can happen with paints. If you put them on thick, or under certain temperature conditions, the outsied layer can dry to a `skin' that traps solvents under it, preventing them from escaping, and preventing the lower layers of paint from completely drying. Try sanding this too early, and you'll go through that skin pretty quickly and make a mess. If you put on thin coats the problem will be reduced, but with each additional coat some of the solvents can be re-absorbed by earlier coats, and be trapped for a long while.

Hope this helps

Paul G. Jacobson