Question about EXRERIOR finishes

Hi all. I have a question about finishes for a wooden boat. I hope it doesn't seem dumb, but here goes... I have seen a few boat projects that the builders have used spar varnish on the exterior. A problem I have noted is that the varnish will "pop" or separate from the wood at times. Would it work (for starters) to add mineral spirits and thin down a "first coat" to allow the thinner mix to better penetrate the wood? Then varnish over with multiple coats for good water sealing and protection. Would this be a good sealing method? My little pond popper has a fiberglass epoxy resin finish. It is tough but high maintenance. I have to sand and re-apply the resin every couple years. I am contemplating building a shanty boat which won't be so easy to maintain the bottom side. I would appreciate any ideas on a finish that is tough and very low maintenance. Thanks. Dave

 

I have had very good results using Penetrol as wood primer. Does a good job of penetrating the wood and sealing as well as a finish in its own right sometimes

 

I tend to agree, Penetrol is useful and versatile, and can be mixed with any turps thinned paint.

 

Finishing wooden elements can get involved, but if done properly, it stays stuck. If it pops off the surface, usually at the corners and edges first, it's simply incorrect technique. Penetrol is a combinations of oils, used for flow promotion and driers. You're much better off using Penetrol as an additive, then as a sealer. Most will thin traditional varnish, by various amounts, but less then 20% is typical, for this sealing coat. Maybe some Japan drier to speed things up, but Penetrol isn't a wise addition as a sealer at this stage. Subsiquerst thinning is only to promote flow or ease stipple from a gun. The thinned coats of varnish are intended to let the oils get into and fill the cut wooden cellular structure, where the vehicle flashes off, leaving the oils to seal the wood fibers. After this sealing process, some species will need to have the grain filled, like mahogany, but other wise, you only need to apply the bulk up coats (a small amount of Penetrol is help here), to generate film thickness. The last stage are the finish coats, where Penetrol again isn't handy and can affect finish quality (dulls gloss), which are applied after the bulking coats have been leveled. These are generally the sacrificial coats, that will be damaged by UV and sanded or scraped away on subsequent recoatings. The subject is well covered in a number of books and one of the best is by Rebecca Whittman

 

How come then it soaks so well on woods and creates an effective barrier? Even in marine environment. Even the can says to use as a sealer for marine use

 

What soaks so well (Penetrol, varnish, thinned varnish)? No varnish or varnish additive is especially effective as a barrier. Moisture vapor transmits right through (okay about 30% barrier if high solids varnish, much less if the cheap stuff).

Alkyd based products don't really seal wood. What happens (again) is the oil borne product (Penetrol, varnish or thinned varnish and even paint) leaches into the wood, because of its vehicle, where it flashes off (the vehicle), leaving the oil to fill the fibrous, cellular structure of the wood. This effectively closes off the cells, so that intermittent moisture exposure can't get in. This said, the wood still acts like wood and changes dimension with environmental moisture content differences (it breaths). In terms of sealing, all a varnish coating does is protects the surface from splashes, it's not really sealed, just protected. Epoxy encapsulation is the only way to seal the surface of wood (97% moisture vapor barrier).

Products marketed as a sealer fall into a few categories, acrylic, polyurethane and modified alkyds. What these do is make a more moisture vapor resistant barrier, by using driers mostly, though the polyurethanes and some of the acrylics actually do turn into a semi permeable plastic when cured, they're still not water proof, just better then a straight oil job.

Sealers are only really useful when you need to even out a color, because the particular species being used tends to become blotchy, because of various densities in the growth rings and grain. This is a step before stain, to even out the tone when varnished. It's an aesthetic consideration and not really necessary.

 

But penetrol is recommended as a rot prevention sealer on the can. Isnt it petroleum based? I have used it on waterfront furniture and on the ply floor for my old PE boat, only that and no other paints or resin. Carpeted with contact glue and it lasted 3yrs that I had the boat. Always got wet too with sea water and fish blood. Sanding after cure shows it soaks well into the surface.

 

What Penetrol is, as I understand it (I've too have used it for decades), a combination of natural oils, adhesion promoters, some solids (more then varnish, less then paint), deoxidizers, some UV inhibitors and some driers. It's promoters and oils are typically better then lesser grade alkyd products. In short it's a "wetting agent", for alkyd products. As a rust and rot preventer, the oils dry tightly enough to prevent moisture vapor penetration under it. It doesn't last long immersed or with abrasion or with UV exposure, but for intermittent exposure, well enough. When used with or under paint, the paint provides the qualities it lacks (abrasion, UV, etc.). Used alone, it doesn't last long at all.

If you want to get technical, its a series of fatty acids (coconut oil for one), mixed with DEA (diethanolamine - a common organic compound) and the addition of other "modifiers". It hasn't any petroleum products in it that I know of, so it's hydrophobic properties, make it just a surfactant mix (emulsifier) with extra features. The way it works is it lowers the surface tension of moisture as it come in contact with it.

In fact, most alkyd paints will have similar chemical combinations (plus other stuff) as Penetrol, just not at that same levels. It has its uses, but as a wetting agent it shines, though not so much as a top coat.

 

DEA is a detergent. Also, it has been banned from soaps and shampoos because it is carcinogenic.

 

This must be new (full up ban), as diethanolamine, though can be harmful to lab mice in the huge doses they receive, but intermittent and casual use in shampoos, cosmetics, and pharmaceuticals it shouldn't cause a problem. Of course some folks will have a reaction to it, but some people are allergic to air too. Lastly, DEA used in it's natural state is hygroscopic, which is why it's used in consumer products, but when formulated in Penetrol, it's hydrophobic. I know the FDA has implemented a voluntarily ban of its use in cosmetics and it's use has dropped considerably since 2000, but they (the FDA) also admit there's no reason for consumer alarm.

 

par thanks for really good info. I appreciate your knowledge in coatings. people don't understand them well

 

Thanks everyone. I appreciate all the info. God Bless. Dave

 

Culler says kerosene penetrates better than anything else. I've used kerosene, linseed oil, turpentine w a bit of varnish as a primer.

He recommends this for the new unfinished boat .. especially inside and the bilge.

 

Thanks Buddy. That does sound like a good primer/finish.

 

Kerosene and oil is a very old school way of doing things, but can't be recommended, for many, mostly obvious reasons on a wooden boat. This treatment also makes using modified polyurethanes and acrylic paints in the bilge difficult, though some primers will stick, leaching oils and petroleum products will always be problematic, unless you stick with traditional coating materials (straight alkyds).

Preventing rot is a function of maintenance, more so then treating the wood with highly flammable solvents. Most of the major manufactures used two methods to protect bilges. The one I think works best, from a maintenance point of view is varnish. The first coats are put down quite thin, but the last are straight (no cut). The first thinned coats penetrate and seal the wood's pores, while the additional over coats offer some film thickness to protect it. The beauty of varnish is you can see right away when a problem needs to be addressed, such as dark stains, where moisture has gotten in around a leaking fastener for example. With paint, (the second option) you can't see this and a problem can go unnoticed, until the paint bubbles.