I have been using a Spar Varnish mix, based on recommendations from several Fine Woodworking authors, including G. Hack. L. Schleining and R. Johnson. These recipes vary from author to author, but essentially say that a little tweaking is fine. This makes sense to me as well, the Varnish adds the alkylds, the oil makes the spar varnish an even longer oil, and the dryers do their work.
Problem:, No matter what recipe I use, I have streaks in the finish when viewed with raking light. The streaks are with the grain, so I am doing something wrong with application or wiping off, right? They do not have any surface texture to them, they look “dry”.
Here’s what I did: I sanded the top prior to adding finish with 600 grit, and it looked great. Denubbed and got all the dust off. I have two coats on now, and can see the streaks. Between coats, I have sanded the finish with 400 grit.
Coats consist of 1/3 McClosky’s Spar Varnish (Red Can), 1/3 Danish Oil, and 1/3 Turpentine, and sometimes Japan Dryer.
Also, any advice on streak removal would be helpful.
Thanks
Ken
Replies
Spar varnish remains spar varnish--a varnish's long or short oil status is fixed in the manufacturing process; adding oil subsequently just creates a mixture of the two substances. Danish oil is already a mix of a different varnish, oil , and (perhaps mostly) thinner, and turpentine is just a different thinner. The Japan drier just confuses the chemistry, possibly impacting certain elements in your mixture more than others.
Sounds like you aren't wiping off the material as you should. Oil varnish mixes, which is just what this is, are too soft to want to leave any film on the surface. (You have just another "Danish oil" with two sources of varnish.) Apply wet, let penetrate for a few minutes only (especially if you have added Japan drier) and then WIPE IT OFF. ALL OFF. This sort of finish just isn't supposed to leave finish to be sanded between coats. You should be just trying to get the wood evenly penetrated so that it has an even, wetted out, look, with no apparent finish. If you still have streaks after wiping it all off, then you had streaks in the bare wood, though your overkill sanding to 600 grit ought to have revealed any remaining scratches.
If you want any sort of film use varnish only, and preferably NOT spar varnish which is also pretty soft and less water resistant than interior varnishes. You can thin any oil based varnish to make a wiping varnish, which you can apply in very thin damp coats, to build any film thickness you want. This isn't a wipe on, wipe off process but one of spreading the varnish very thinnly and evenly. You can apply up to three coats in a day, if you apply them just after the preceeding coat has dried enough not to be tacky. (Sanding not needed (or really possible) between these coats.) Then the set of coats should be allowed to cure overnight, and then scuff sanded before adding additional coats. Those bunched sets of coats crosslink enough that will be able to rub out the finish without witness lines between those coats.
Steve ----
What's the function of Japan Drier? As the name implies, can it be added to finishes (either varnish, or vanish mixes) to speed drying?
Japan Drier
Japan drier consists of metal salts, usually cobalt, in an appropriate solvent. It acts as a catalyst to speed up the curing process, which I understand is oxygen dependent polymerization. It will speed up the rate of cure, but is not necessary as varnishes, boiled linseed oil, alkyd paints etc., already contain some drier. The effect can be seen by comparing the drying times of 'boiled' vs raw linseed oil and pure vs 'polymerized' tung oil.
Japan drier adds an additional dose of the metallic salts that can accelerate the curing process. But, since you don't generally know which particular driers the manufacturer of thede BLO or the varnish has used, or for that matter you seldom know which drier or driers the Japan drier contains, you run the risk of having too much drier, especially with some versions of the "hot" finish. Too much drier can result in brittleness and reduced adhesion. At the best you would be increasing the speed of the cure a little, but without really knowing the chemistry you can't really predict that speed increase accurately and shouldn't add additional coats, etc much ahead of the normal schedule anyway. Just doesn't make much sense for a routine addition to a product mix.
Alternatively you might try a mix of 1/3 varnish (spar or non-spar), 1/3 boiled linseed oil and 1/3 mineral spirits. A local woodworking store owner said that is the general content of danish oils. His additional recommendation was to use a bit less boiled linseed oil if using spar varnish. Wipe on, then agressively wipe off until a clean towel doesn't pick up any more.
Aggressive Sanding
The varnish mix to which you refer is intended as a penetrating finish. By sanding to 600 grit, you may have burnished the surface to the point where the pores are sealed and the finish cannot penetrate. Before your next application, rough up the surface by sanding 120-150 for hardwood and 180 for softwood.
Don
This is simple, use a 3M pad equal to 600 grit and go with the grain like the 600 sandpaper, but after the sandpaper. The sandpaper leaves the streaks because you are hitting the high points only. The 3M pad gets into the grain as well and finishes the process equally. I apply the finish then wet sand it with a 3M 600 (equal) pad, lightly with the grain, let sit until ready to wipe, and wipe dry. If I do this for each coat, the newly applied finish slightly softens the previous coat and a light rub with the 3M pad removes any errors as well as etches the previous coat improving adhesion. I only use dry sanding for brushed or sprayed coats to flatten and I use the 3M pad, dry, to even it all out, but the coat must be hard enough to to avoid tearing the finish. You can even go to a white 3M pad, the super fine, as this will remove even less finish on the wiping type finish, allowing you to build the finish faster.
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