Rubbing out a polyurethane finish


  #1  
Old 02-09-02, 07:36 AM
mkorsten
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Rubbing out a polyurethane finish

I have brushed on several coats (sanding with 320 grit between coats) of a polyurethane finish (Minxax, fast drying polyurethane) onto a dining room table I am refinishing. Unfortunately, the finish has imperfections (tiny raised bumps) which I attribute to residual sanding dust and cat hair (I have 4 cats roaming around the dining room area) rather than trapped air bubbles. I have lots of experience rubbing out lacquer finishes (I build classical guitars in my spare time) with finer and finer grits of wet-dry sandpaper (and lastly, buffing). If I wait the usual several weeks to allow curing, can I do something similar with polyurethane or am I asking for frustration given the intrinsic chemical differences between lacquer and polyurethane?
 
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Old 02-09-02, 08:20 PM
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Rubbing out a poly finish can be done, but as you suspect, there's a hazard built in because of the difference in the finish.

Lacquer is an evaporative finish. The second (and subsequent) coat(s) remelt the coats preceeding it, allowing the finish to flow together.

Polyurethane is a reactive finish; the finish is actually in layers. This is the reason sanding is required between coats; you need to rough up the surface to give the next coat something to hang on to. It is, however,easy to sand through one layer, which results in a ring that's difficult to conceal.

If the last coat was fairly heavy, I see no reason why someone with your experience couldn't rub out the finish.

I'd start with 600 grit. It'll take a little longer, but you're much less likely to 'burn through' the top layer.
 
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Old 02-11-02, 08:53 PM
keldon
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Polyurethane

I am using Minwax Fast-Drying Polyurethane-clear gloss on an oak table. I'm getting small bumps and dimples when it dries.
I'm also refinishing other items with no problems but I'm using
Minwax semi-gloss or Red Devil. I think there is a problem with the Minwax clear gloss.
 
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Old 02-12-02, 07:31 AM
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keldon,
If 'fast-drying' means it is a water base poly, the water in the finish is raising the grain. You need to sand it lightly with about 320 grit paper and apply another coat(or two) of finish.
If the 'fast-drying' is an oil base finish it simply is drying too fast to level out properly. Pause. I just went down to the 'shop' and found that both the Minwax polycrylic and oilbase poly cans say "fast drying". I guess time is relative.
Oak is nearly impossible to get a really smooth finish on due to the graininess. (Grain fillers are a completely different story).
I suggest to thin the oil poly down with mineral spirits - it will glide on easier and have more time to level.
fred
 
 

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