Sand-through with water borne lacquer

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Re: Sand-through with water borne lacquer

Postby rob newell » Thu Feb 01, 2018 6:25 am

Mike, you say:

As Gordon says, the 400 grit sand is to knock down higher spots or brush strokes, and take care of any runs. With dry sanding and light pressure you can see where the high spots are getting sanding, and I try not to sand past when everthing looks dull.

Do you mean that between coats it is OK to leave areas of unsanded, shiny finish showing?

Cheers

Rob
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Re: Sand-through with water borne lacquer

Postby Steve Sawyer » Thu Feb 01, 2018 12:08 pm

Rob - that was my interpretation, and is the practice I followed on my latest effort. It certainly seemed to make no difference in the quality of the pre-final-sanding finish; each subsequent coat buried the shiny spots, and the next sanding revealed new ones. When I start the final sanding next week, the low spots from the final coat should disappear without any problem, if not with the 800 grit, then definitely at some point before I get to 2000.
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Re: Sand-through with water borne lacquer

Postby Brian Evans » Thu Feb 01, 2018 1:53 pm

With a non-catalyzed or pre-catalyzed finish (most of the waterborne, one part finishes are one or the other), the new coat of finish will chemically bind or "burn in" to the previous coat of finish if you recoat within a few hours, or a day. Sanding is not required for that, and may even be sub-optimal in terms of building the layers of finish to a depth that you want to achieve. With a totally non-catalyzed finish like old school nitro-cellulose, a subsequent coat can chemically bond even after years of time because the finish never cures, it only drys. With a pre-catalyzed finish (like the Brite-Tone instrument lacquer I have been using) the finish drys in an hour or two, but does not "cure" for several days. If you re-coat before the cure has begun but after the coat has dryed, the chemical bond happens and the new coat burns in to the old. If you re-coat after the finish has cured (the catalyst activates, the finish chemically changes and cross-links the polymers, as I understand it - not a finish chemist!) then the new coat will not chemically bond but will only mechanically bond. Like epoxy, at that point you need a rough surface to let the new coat have mechanical teeth to grip to, and I would personally use around a 180 grit at the finest.

If the finish you are using has words like "pre-catalyzed", "cures" and "cross-links" in the description, most likely you have a pre-catalyzed finish that cures after several days and is problematic to re-coat after a full cure. I am not sure, but I think the finish that started this conversation - GF High Performance Top Coat - is not pre-catalyzed. They discuss re-coating after many days, top-coating many different kinds of old existing finish, and they sell an equivalent finish that is actually named "Pre-catalyzed". That does not necessarily mean that it can chemically bond with itself after many days, months or years like nitro-cellulose can. I find most finish manufacturers are far more interested in selling you something, anything, than selling you the right thing and educating you on what it is and how to best use it.

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Re: Sand-through with water borne lacquer

Postby Barry Daniels » Thu Feb 01, 2018 2:19 pm

I use nitro and sand about every 6 coats. Each time I sand I see fewer low spots. I know that there are some folks who hold off on sanding until all coats are applied. But I think sanding during finish application results in a more level finish and less chance of sand through.
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Re: Sand-through with water borne lacquer

Postby Mike Conner » Thu Feb 01, 2018 2:20 pm

rob newell wrote:Mike, you say:
As Gordon says, the 400 grit sand is to knock down higher spots or brush strokes, and take care of any runs. With dry sanding and light pressure you can see where the high spots are getting sanding, and I try not to sand past when everthing looks dull.
Do you mean that between coats it is OK to leave areas of unsanded, shiny finish showing?
Cheers Rob


Rob,
I look at the small remaining shiny spots as thinner areas, and that the 400 grit sanding is bringing the thicker areas down to that level.

As Brian has stated, the waterborne finish should chemically bond with the unsanded areas as long as the additional coats are applied before the cure has happened. You could always buff the shiny locations with a super-fine abrasive pad if you feel the gloss surface is a risk - but I haven't taken it that far.
//mike
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Re: Sand-through with water borne lacquer

Postby rob newell » Thu Feb 01, 2018 6:55 pm

Thanks, to Mike and others who replied to my query. I appreciate it.
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