Polyurethane Questions

[Mark Kelly]

Anyone ever use oil-based Pro-Finisher from Parks? I am looking for a non-yellowing polyurethane, and I think this may be the best one to use. Would love to hear suggestions, especially as to how well it ages (its non-yellowing aspect).

As a side note, how does one thin waterborne polyurethanes?

[Chris Padilla]

Mark,

No experience with that product but typically, whatever they suggest for "clean-up" of the product is what you can use to thin the product. Hopefully the can has some info about thinning (like for spraying) on it.

[Jim Becker]

WB products generally cannot be thinned. You should by the formulation required for your intended application method...there are WB urethane finishes available for spray application (Target Coatings has one). But there are also a lot of other finishes that offer very good "non-yellowing" characteristics, especially in WB, with better clarity than the "evil polyurethane" finishes. Poly is the most over-hyped product on the finish shelves, especially at mass marketers and in most cases, doesn't "perform" any better than a variety of other finishes that have nicer features. For example, soya-based finishes, such as Pratt and Lambert #38 are much lighter in color than BLO-based varnishes, causing less of the yellow cast that you cannot avoid for an oil-based finish. I pretty much use Target Coatings Oxford USL acrylic lacquer (WB, of course) for my spray finishing...easy to use, great finish and very low VOC. (much safer that way). Traditional solvent-based lacquers also retain their clarity and offer less yellowing than oil-based varnishes.

[Mark Kelly]

Yes, I agree that polyurethane is overhyped, and for most of my stuff I finish with traditional BLO and/or shellac. However, in a recent article in FWW there was a method for wipe on polyurethane finish. I was intrigued by the wipe on method (as this is what I prefer and I have no spraying equipment) especially in using it for kitchen table-top surfaces. And since I am not brave enough to try something that hasn't been tried before, I depend on these articles or help here on ideas for finishing (as well as other topics.)

So, maybe my question should have been, are there any type of wipe-on finishes (or those that can be thinned so as to be able to wipe on) that are super durable and non-yellowing?

[Howard Acheson]

There are a number of suggested application regimens that are totally subjective. The number of coats in a given day, the % of cut on various coats, which coat to sand after, when to use the blade and a whole host of other practices are all minor differences between finishers. There are some things that I consider sacred when applying a wipe-on finish.

If you are making your own wipe-on the mix is scientific - thin. I suggest 50/50 because it is easier to type than any other ratio and easy to remember.

The number of coats in a given day is not important. Important is to apply a wet coat with an applicator and merely get it on. Think of a pimply 16 year old kid working as a busboy at Denny's you have sent over to wipe off a table. The applicator can be a paper towel, half a T-shirt sleeve or that one sock left after a load of washing. Then leave it alone. If you have missed a spot, ignore it - you will get it on the next coat. If you try and fix a missed spot you will leave a mark in the finish.

Timing for a second coat involves the pinkie test. Touch the surface with your pinkie. If nothing comes off you are ready for another coat. It was tacky 5 minutes ago but not now. Apply your next coat just as you applied the previous coat. Remember, you are wet wiping not flooding. After applying the second coat, let it fully dry for 48 hours. Using 320 paper and a sanding block ligthtly sand the surface flat. Now, begin applying more coats. Do not sand between coats unless you have allowed more than 24 hours to elapse since the prior coat. Keep going till you are tired of it. The number of coats is not critical - there is no critical or right number to apply. For those who need a rule, four more coats on non-critical surfaces or six more coats on surfaces that will get abraded seems to work.

After your last coat has dried at least over night you will have boogers in the surface. You should not have marks in the surface because you ignored application flaws. You will have dust, lint and, if you live in Texas, bug legs. Use the utility knife blade at this point. Hold it between your thumb and forefinger, near the vertical, and gently scrape the surface. Gentle is the important word - no harder than you would scrape your face. If you start scraping aggressively you will leave small cut marks in the surface. After you have scraped to the baby butt stage gently abrade the surface with 320 dry paper or a gray ScotchBrite. Clean off the surface. Apply your last coat with a bit more care than the previous coats and walk away.

An anal person is going to have a tough time with this process. Missed spots have to be ignored. Wet wipe, don't flood. Scraping to babies butt smooth means scraping no harder than scraping a babies butt. Ignoring any of these will leave marks that are tough to get out. Getting these marks out requires some agressive sanding to flatten out the surface and starting over.

Jim Kull

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[JayStPeter]

As a side note, how does one thin waterborne polyurethanes?

Jim is correct when he says to buy WB for the application method. But, I read a bunch of spraying info from Jeff Jewitt and he said to thin WB poly with milk. You can't thin it much though. I haven't tried it personally.
Try looking at the articles at homesteadfinishing.com.

Jay

[Mark Kelly]

Thanks for the tip. Jeff Jewitt has an article that says you CAN thin a waterborne finish with 10-20% water. Here is the link:

http://www.homesteadfinishing.com/htdocs/Wiping%20Waterborne%20Finishes.htm

[Jim Becker]

But if you use the right material for spraying and have the correct projector set in your gun, you shouldn't have to thin the finish.