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Topcoat Brush

D

djg

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Dan
I'm not very good at applying a topcoat to my projects. Mainly Poly Urethane (PU). I recently replace the top on a utility folding table with a piece of plywood. Then I finished it with just PU. Plenty of bubbles and some lines. My projects so far are only 'utility' and not furniture. So, the finish really doesn't matter. I'm not going to get bent out of shape trying to get the best finish on these projects. But when I do something, I do try to do the best I can, treating it though as if it were furniture. If for no other reason for the practice.

I use to us oil based PU and don't remember having major issues. Lightly sanding with steel wool between coats was all that was required. But I had my oil based PU drying up in the can (last). So I switched to water base. Same brush. I use a Wooster "Badger hair" brush. Maybe since oil based PU dries slower, it allows the surface to level out.

Anyone have experienced the same problem with water based PU? What brush do you use?
 
For water base poly I've found stirring more aggressively than a gentle touch will put more of the bubbles in my work. Having a hair dryer nearby for some focused heat with low air volume on the bubbles helps as well.
 
I've been using Formby's tung oil (not the 100% stuff) and Minwax wipe-on poly for most of the little projects I make. And I also struggle getting a glassy finish. One thing I've found that helps is to sand the project to 320, and after the first coat of finish with 320, not super aggressively, just until the surface feels smooth to the touch and I don't see or feel any rough spots. Apply a second coat of finish and then rub with 0000 steel wool, again not super aggressive, just until I get the sheen off and don't see any rough spots. Those first coats just need to get enough on to cover. The last coat however needs to completely cover so the surface looks uniform and wet. Not so much it runs but more than the first two coats )or maybe it's about the same but the first coats soak in more). It's kind of a fine line to get it right. But it's enough so that the finish will self-level. If I skimp too much with the finish it doesn't have enough to level and I get the streaks you mention. I have also found I need to move along, get the final coat on there and don't over brush, try to get a single pass in a slow smooth even motion, to get the cover you want. It might mean you have to refill your brush or rag more often, after every stroke or two. Trying to overload the brush and making more strokes doesn't help. Especially on warmer days, the finish dries too quickly and running the brush over it a second or third time puts streaks into it. I check the application by looking at the light reflection because if I look straight at the surface I often miss dry spots. Oh, and the same things apply regardless whether I use a brush or rag to wipe on the finish. For me it's easier with a rag and wipe-on poly.

Regarding bubbles - I've had best results with wipe-on poly rather than using a brush. A small square of old t-shirt folded into a pad seems to work best for me.

The type of brush doesn't seem to matter, I can get a crappy finish with any of them. The amount of finish and the speed / efficiency I use to apply it seem to give me the best results. Hope that's helpful.
 
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I'm still experimenting myself, but one project I tried using a sponge brush with great luck. Bubble-free finish! I do agree with Jonathan on not stirring aggressively in the can. Chuck
 
Another thing I've used recently that seems to improve the final appearance is after the final coat of finish has thoroughly dried, I wipe it gently with a gray micropad thing. Not sure what it is exactly, it came as a sample to try in another order. But it works well. It just dulls the final surface and then I rub Minwax paste wax onto the surface and buff it off. I seem to get a more uniform sheen doing it that way. My guitar coach does something similar except he uses car polish wax.
 
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I think maybe I put the finishes on too thick since I see streaks bare spots. That may have something to do with the leveling problem. I have tried those foam brushes and in my experience (limited), I have more problems with the brush leaving bubbles. Never tried a rag, though.
 
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Several years ago, I swore I would never use water based finishes, or those cheap foam brushes. Then one day, when I was out of brushing lacquer, and didn't want to set up my spray setup, I was working of a small project and decided to try Minwax Polycrylic Satin and a cheap foam brush....
That was more that 10 years ago,, I still use the same products.....
The process is, 1st sand the wood to 220 grit, apply 3 or 4 coats, let it cure for 2 to 3 hours between coats. Sand again to 22 grit between coats.
2nd, apply at least 4 coats, sanding to 330 grit between coats, let cure for 5 to 6 hours between coats.

After a day or more, I wet sand, using 800 grit first, then go to 1200 grit. Wipe off the grit, then use a white 3M pad to polish to the final sheen that you want. On a few projects, I've applied as many as 10 coats for a really deep, durable finish...I use this finish for urns that I build....
 
That ought to do it Larry. With that many coats, in the end you're finishing/polishing the finish instead of wood.
 
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