Two pot polyurethanes in seawater

[Ric Fallu]

For some time I have been crafting wooden spear guns. I normally finish the wood with two pot polyurethane floor varnish (Estapol). (Woods can be various, commonly I use meranti, radiata pine, jarrah, Tassie oak, mirbau and Huon pine. Often a mix laminated).

The polyurethan doesn't have a lot of UV resistance, but it is bloody hard, and resists scratches in the boat and on the reef. Once successfully applied, it seems to last well enough in a difficult environment.

(I have toyed with the idea of using the International two pot polyurethane - it is formulated for a marine application after all. But so far, have yet to actually try it.)

My problem is, I sometimes find difficulties in getting the finish to look real good. I take trouble with sanding the stocks, usually going down to 120 grit on the wood. Then I put on a coat of varnish, and take it down with 120 grit sand paper. It seems to take 4 to 6 coats to get the grain filled (depending on the wood).

Its getting that last deep glossy coat that sometimes eludes me. If I flow the varnish on thick, I get runs (usually coming from the holes and indents in the stock but sometimes just because I put too much on). If I don't flow it thick, the gloss looks thin and patchy in places. My common practice is to try a coat: if its no good to sand and try again, but this takes a bloodly lot of trouble. The last time I put on 10 coats but still didn't get a goodie before I gave up. (After I gave up on the varnish, I spent a bit of time with floor wax - which will retain its shine for about 20 minutes under water - but it looked OK for a while)

Yet sometimes, I put on a coat of polyurethane that looks thick and lustrous, without too much effort at all.

I use a brush. I have tried expensive brushes and I have tried cheap brushes. The main difference seems to be whether or not they shed the occcasional hair.

I have also thought about getting a sprayer, but haven't really gone that far yet. Deciding between an airless and a compressed air gun has been too complicated so far.

I have also considered setting up some kind or rotisserie to rotate the gun whilst is dries - but this would involve a lot of effort and I wonder about the marks where the gun would be held.

I have also been toying with the idea of using epoxy or even marine varnish - when I have worked on boat parts, the latter seems to go on really thick and lustrous, but seems soft. I suspect the really good looks from marine varnish would last about one dive or two, then all you would see is a mass of scratches.

Anybody got any advice to give on varnishing wooden spear guns?

[Shane Watson]

Basically you'll find everyone telling you to sand to a finer grit....And thats the very best place to start looking for a possible cause. 120 is far to corse a grit to be using inbetween coats. Most people will also say its far to course for the final sand before the first application of finish and I agree.

If I were you I would at least sand to 240g before finishing. Then depending on how well the finish sands you should be using anything from 400g up to 1200g inbetween coats.

PREPERATION IS THE KEY TO SUCCESS.

But this may not be your problem, your problem may lie with finishing in a dusty environment (even if you think it is fairly dust free it may not be) Contamination of the finishing product. Finishing product to thick or drying to quick.

It is always better to apply many thin coats as opposed to one heavy coat.

Ideally spraying would be your best solution. But if your only doing the odd one here and there it may be too costly to setup. But once you have setup a spray system (even a very basic one) you'll have it to use on much more than just finishing timber.

Cheers!

Shane.

[Shane Watson]

By the way I just flew out of Darwin yesterday (thurs) where abouts are you in the NT?

[journeyman Mick]

Ric,
what Shane said, plus gloss level is determined to a large part by drying time. In a hot humid environment a two pack poly will go off very quickly. If it goes off before it has had time to flow and flatten out all the bumps caused by brushing you won't get a good gloss. Also in some cases the top surface will go off before what's underneath, especially of you're trying to "flow" it on to get a higher gloss level. This will give you a slightly flat finish (and in extreme cases a white "bloom"). I'd look at doing some testing on offcuts rather than all that tedious sanding back of your stocks. Get a stack of offcuts and number them and sand them to at least 240. Give them all a first coat and sand again. Mix up a measured pot of brew and coat the first piece then add a small amount of thinner/retarder to the brew (use a teaspoon or similar for accurate measuring) and coat the next piece, continue coating and diluting your brew (there will be a range of concetration/dilution reccomended for the retarder) and check out what mix works best. Don't know what it's like in your neck of the woods but it's getting dryer and cooler here now so what works now may not be the best in November/ December.

Mick

[Ric Fallu]

Shane

I am located in Darwin (though I travel a little with my work). I reside in Stuart Park, and that is where I do the wood work. I make about 10 spear guns a year. The NT is the place to go for fishing, and the water is warm enough to swim in without a wet suit - of course there are stingers, crocs and sharks, but that only evens up the odds.

One of the problems I have been addressing in the NT is humidity - its getting better now that the dry season is with us, but it certainly makes wood swell, and when you are laminating, different woods will swell differently of course. Its essential to balance the different laminations.

I did have an orange peel problem with the finish, and that may well have been the humidity. Another problem is that silicon grease is used with the rubber parts of spear guns, and it is easy to mistakenly use the same rag to clean off sanding dust. I also coat saws, spade bits etc with dubbin to prevent rust and that can be moved around inadvertently. Cleaning with white spirit btween coats seems to help.

Back to sanding - I try to make the spear guns of a standard better than rough, but they aren't furniture, and I wonder if cabinet making standards (eg 1200 grit paper) are applicable. They get pretty rough treatment when in use. I can sometimes get the results I seek with 120 grit (with the white abrasive), so why can't I get the results other times? I do my rough shaping with rasps, 60 grit, etc and the 120 grit seems to remove all visible scratches. I am not going into denial about your advice, and will try what you advise on the current gun, but I just wonder about the variations for no apparent reason.

Ric
Gunner of the North