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I’m using satin polyurethane thinned with about 10% mineral spirits. I find that the varnish is drying with brush marks still showing. I’m using a good quality bristle brush, flowing it on, then tipping it along the grain. I suspect the problem may be related to the recent hot humid weather; even though I wait 24 hours before recoating, it’s possible that the previous coat has not cured enough. Would this accelerate setting of the new coat enough to cause drying before the brush marks level out?
Thanks for any help understanding this problem.
Crawford Woodman
Replies
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Crawford,
I suspect that it is a combination of high humidity and the thinning. Perhaps you can provide us with the brand name of the poly, the temp and humidity during application. Is this a china bristle brush you are using.?
Dano
*Thanks for your prompt reply, Dano.I'm using Flecto Varathane, which has worked well for me in the past. My workshop and finishing room are in the basement, so slightly cooler than the 90 deg.F temperatures outside. I have a dehumidifier running almost constantly these days, but it still feels a bit muggy. Also, to combat the varnish fumes, I have an exhaust fan in the window (so outside air is being drawn down into the shop).The brush I'm using just says "Fine Bristle" on it.When I lay down a new coat, it flows on well, and for the first few minutes it seems to be smoothing out just fine. But when I look again a couple hours later, it has set with traces of brush marks still showing. I guess my basic question is whether not waiting long enough for a previous coat to cure can cause a new coat to set too fast (before it has a chance to level out the marks)?Crawford Woodman
*Crawford,I would recommend that you get a China bristle brush, Linzers and Purdys are pretty good. I respectfully disagree with T on loading the brush with mineral spirits first. By doing so you have effectively lost "control" over the varnish. I sometimes rotate between two brushes. When I change I always make sure that the "new" brush is completely dry and free from solvents.Also, could it be possible that you have not been sanding between coats? It also sounds as if the humidity is really pretty high. Use the fan as an exhaust rather than an intake.Dano
*Thanks, Tab.I have been pre-wetting the brush with thinner, flowing the varnish on quickly and tipping off right away. It worked fine on the first coat, but on the second it left brush marks. I waited for it to harden, then sanded the whole thing smooth with 120 grit on a random orbit sander, cleaned it off with a vacuum and tack cloth, and tried again -- same problem.Your suggstion of trying different ratios of thinner sounds like a good one. I don't think my client will be happy waiting until fall.Crawford.
*If you can, you might try using high gloss for all of your base coats, and then one final thin coat of satin. The satin has to "set up" faster than gloss to keep its silicates from settling to the bottom of the layer and then looking glossy. The added advantage is that the finish is more transparent with only one layer of the cloudier satin.If you're sanding between coats, you may also be getting some sandscratch swelling where the uncured poly will absorb the solvents along the lines which have been abraded by sanding, swelling with the solvent, then sinking when dry, revealing a rougher surface. A longer cure between coats would take care of the problem.
*Thanks for the advice folks. All this trouble with brushing has made me wonder...have any of you tried brushing on the first couple coats, then sanding and WIPING on the last couple? Would this not give the best of both worlds: a thick durable layer, with an easily applied final coat free of bubbles, dust (and brush marks!) I've read many articles discussing the relative merits of wiping vs. brushing, but have never seen anyone discuss combining the two.Crawford.
*Thanks, T. You're right, there's a technique to wiping on a finish, and I don't have it yet. I've been experimenting and sometimes it works OK, and sometimes it doesn't. Biggest problem seems to be lint left by the cloth, even though I chose the most lint-free rags I could find. Tried paper towels, as suggested in a FWW article, but no-one on town has the unpatterned kind, so that didn't work too well either. One advantage of the method seems to be that if you don't like the results right away, you can just wipe off the whole mess and try again.Crawford.
*Crawford,If lint is a problem, use linen or muslin.Dano
*Thanks for your help, guys. I increased thinner to about 20% and it flows on nicely leaving no brush marks! Also picked up some cheesecloth at Lee Valley which is supposed to be lintfree; I'll try it next time I wipe on a finish.Crawford.
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