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I seem to have a re-occuring finishing problem in which I need assistance.

Let me first describe the process:

2 applications of waterbased grain filler – sanded.
12 coats Stew-Mac Waterbased sanding sealer with color added
8 coats of Stew-Mac waterbase top coat without color.
I end up with a very nice matte finish. No problems appear anywhere.
Sand with 1200 grit, 1500 grit, and then 2000 grit. The finish still looks good.

Then I go to the pedestal buffer to remove scratches from the sandpaper and acquire the nice shiny finish.
This is when I start to notice a “blush” on the finish. The buffer is creating a blush? The surface of the instrument looks continuously smooth, but when held at another angle, a blush appears. I got a very similar pattern from two instruments, so I concluded it was my buffing technique. These blushes appear on the headstock top, the body back, and body top. The blushes are NOT at the edges, but in the middle of the considerably flat surfaces.
My theory is this: I buffered through the clear topcoat and got to the colored sanding sealer coats. I’m too careful on the edges when I buff, but not careful enough elsewhere.

First I’ll share my thoughts on the repair: I need to apply additional top coats and be a little easier on the buffer.
Secondly how to avoid this in the future: Limit number of color coats to the first few. I should not have placed 12 coats of color sanding sealer on the instrument.

Does this sound like a good assessment of my results and mistakes?

What color pigment are you adding? Translucent or opaque?

I seriously doubt that the buffing has any thing to do with the bluish cast you speak of. That is a tendency of waterborne finishes and sealers in general. The darker the wood the more noticeable the bluish tent tend to be.

You did not say how long you allowed the finish to cure out before buffing.

Michael, he said nothing about a BLUE blush. It very well could be sandthrough to the sealer coats. I think Stew-Macs finishing schedule of that many sealer coats is back asswards.

You could have a couple of things going on:

First, I'd say you have way too many coats of sanding sealer. I don't know much about the Stewmac WB sanding sealer, but some of that stuff is thick--a lot thicker than the topcoat.

Second, you could have your sealer and topcoats on too heavily or not allowed enough time between coats. Get too many heavy coats without enough cure time between, and you trap moisture, causing hazing. This may diminish over time, but given all the coats of sanding sealer, you may still end up with a blue cast.

Third, did you sand the pore filler back to raw wood? If you left any of that WB filler on the surface, that could contribute to your blue haze.

I am believer that a good finish is possible using waterbornes, but one needs to keep the coats thin, allow proper cure time, and keep the overall finish level thin.

Ken

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http://www.casperguitar.com

Even Stewmacs "Guitar Finishing Step-by-step", in the article "8-day schedule..martin style finish using waterbased..", says "not counting wash coats, spray 12-16 coats of sanding sealer...".
Never liked that! (Also, never tried it.)

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Dave
Milton, ON

+1

many sanding sealers have added solids similar to the flattening agents in top coats...this makes sanding of the sealer coats a lot easier..

with that many layers of sealer that's an awful lot of additional stuff in the mix, which is very possibly what is causing this blush effect being mentioned...also, a sealer is a sealer, its not intended to be layered up real thick, that is the job of the top coat...I run by the rule of no more than 2 coats of sealer and that is only if I am going to level sand a fair bit before I start applying the top coat...

Michael: It’s a translucent pigment. Stew Mac brown.
I let it cure for 7 days before I started buffing. Though I must say the humidity has been high. The instruments have been in a closed garage which is little better than ambient. I have a hygrometer and only lacquer at ideal times.
The blush does not does not appear blue-ish in my opinion.
I followed the “8-day schedule” in the “Guitar Finishing Step by Step” just as Dave Stewart mentioned. I did everything Dan and Don said. In the past I have never sprayed that many sealer coats, but please note that this “blush” is not the first time it’s ever happened.
For reference, and I know there are more variables but, I used one quart of sanding sealer on 3 complete soprano ukuleles. Stew Mac says a one quart can will do 5-6 Ukuleles, so it must be little thick. In the past I’ve put it on so thick the masking tape at the bridge could only be seen, not felt. This time was nowhere near that thickness.
Ken C: You’ve got some good points. The sanding sealer could use a little thinning in my opinion also. (Though finishing only about 15 instruments over two years does not really give me a strong, educated opinion.)
Ken, your second point made me think. I have one of the first ukuleles I finished for my daughter a year and half ago. I picked it up the other day, and it has one of these “blushes” on the back. I saw it when I finished the instrument back when, and it’s still there after a year or so. I don’t think it’s going away.
I did sand the filler back to raw wood, though sometimes I think I’m just applying the filler and wiping it back off.
The funny thing is this: I do not see these “blushes” until I start buffing. Should I be able to see them on a matte finish?