The Ballad Exchange

As suggested by Bob, I have started a separate thread re my embattled locker lids.

I stripped the varnish off with heat and a scraper and set about trying to remove the black bits with oxalic acid, this failed, so I moved on to bleach, and finally I tried mould killer. None of the above really worked to get rid of the black bits entirely, so I gave up, washed the bleach off with white vinegar, washed with soapy water to get rid of the vinegar and finally wiped with acetone, before epoxying them (with a view to varnishing over the epoxy).

The nasty end result:


The 'other side' where I didn't use the chemicals and where it looks cracking:


The remaining black patches after the epoxy was removed :


Bob commented:

Yeah, I thought they were ply. In which case, ignore the grampuses on WoodenBoat advising you to put them thru a thickness planer! You likely have acid stuck down in the face-veneer surface checking (probably in the form of crystals) which will be very difficult to get out. Epoxy is basically a phenolic resin cured with an amine crosslinker, which is moderately basic (alkaline) and modifies the pH of the resin, aiding polymerization. So you can guess what the introduction of an acid to your surface will do for localized epoxy curing! Likewise, superglues (cyanoacrylates) cure in a basic environment provided by oxygen + isocyanate catalyst. CA glues will never cure on certain acidic woods like oaks. You can force or accelerate its cure via an amine dryer in a spritz bottle, or even rub baking soda over the repair.

So you may need to neutralize the acid first with baking soda, ammonia, or lye (sodium hydroxide) solution. You could also try to seal the surface checks using dewaxed (Super-Blonde) shellac, sanded back; or try a penetrating epoxy sealer, like Smith's, which is heavily solvented and less sensitive to the presence of mildew, acids, waxes, or other contaminants than WEST is. Nasty, nasty stuff to breathe, but incredibly effective. You'd still want several coats of exterior-grade varnish (probably solvent-based, for compatability) to protect the epoxy from UV damage.

http://www.smithandcompany.org/CPES/

In response to that I started responding on the other thread with : You know your stuff! I have tried sanding down a little way to remove contaminated wood and it has improved somewhat, I will tiptoe forward with the sanding - I am now super cautious applying anything else to the wood that might be incompatible with the West System gollop (I've currently spent all my boat funds on trying to get this flipping stud out of the engine (which is still stuck btw http://www.ybw.com/forums/showthread.ph ... off!/page9)).

If I can get rid of the patches through sanding, I will be happy - it won't be a perfect finish on the lockers, but it'll look pretty damn good if I can get it to look like the other side.

Before sand:


After sand:


Hummm

Nice job, Mark.
All the solid mahogany trim pieces in Pagan were blackened from moisture, so I soaked them (for weeks!) in wood bleach and they came out better than ever. However, after a couple of years the black started to reappear in certain areas. At first I was miffed, but it gives it a rather nice patina, and as they are the only pieces left from the original interior they can stay that way.

One thing I should have mentioned in the other post: be careful with lye (sodium hydroxide) or hydrogen peroxide as wood bleach; they will remove stains nothing else will, but in stronger concentrations can remove the native color of the wood! Oxalic acid is the most frequent over-the-counter 'wood bleach' and is safer, as it mostly removes mildew and iron oxide staining w/out overly changing the wood's color. However, due to the doghair grain of many mahoganies and mahogany-emulaters like sapele, and due to surface checking of veneers (inevitable with rotary-cut plywood), that acid can crystallize in the checks or the grain & interfere with proper curing of epoxy.

The safest regimen for your hatches might be: oxalic acid and/or vinegar scrub, soft bristle brush with the grain. Rinse. Chlorine bleach diluted 2:1 with water, lightly brushed. Rinse. Then a fairly strong baking soda (bicarbonate) solution, left in contact for several minutes. Rinse a couple times. Let dry very thoroughly. If possible set the wood in the sun for several days, turning over occasionally. (I know, I know: England in November.) You want all the lurking moisture to gas out. The surface will be knappy, like suede leather due to raised grain. Sanding with 180-220 grit paper on a slight bias to the grain will shear off the whiskers.

Now you face the tricky question of what to refinish with. I personally am not a big fan of marine epoxy as a sealant or base coat in outdoor apps, especially horizontal surfaces. Not unless it is painted over. Even the best spar varnish lets some UV reach the epoxy base coats, and they will degrade over time. Might be two years in Wyoming, ten years in Northern Europe. But when they fail, you have an ugly stripping job on your hands. You could begin with either a color coat or a CPES like Smith's; the exact order of the first coats may take some experimentation. A color coat will help disguise localized blotching, and if matched closely to the wood color, need not look gaudy. Pigmented stains will provide some UV protection, at the expense of blunting that lovely shimmer. Many translucent/transparent drying oils (Sikkens, Cetol, Watco) carry some pigment load in the finish itself; better UV performance, muddier appearance.

Then for topcoat, you have the eternal choice between a thick film coating (harder application, better performance, longer life, harder to repair) and a thin penetrating finish (easy application, less physical protection, need for frequent touchups, easy to repair). In your climate, varnish makes sense. In desert Mexico, where we are planning to park, it is suicidal. You could brush on 8 coats of Epifanes, and in half a year all 8 coats would turn to powder and blow away. Better to use a penetrating varnish or oil & just plan on recoating it four times a year. Oils can blacken over time in humid climates.