Clear poly repair

[Ron Overs]

At 7:56 AM -0600 3/1/03, Paul Chick \(EarthLink\) wrote:


>Are the properties of the 2 part polyurethane sealer much different than
>polyurethane topcoat?

Quite similar, but the undercoat is easier to sand, and in the case 
of the material we are using, it is quite brittle and prone to 
bridging if applied too thick.

>   Will polyester adhere to both equally well?

Yes but the polyurethane should ideally be sanded lightly prior to 
pouring the poly topcoat, otherwise the poly may not key into the 
polyurethane adequately.

>Clever idea using the polycarbonate film.

It helps.

>
>  I have had test repairs come out a little lighter under the poly, and I
>learned that the amount of hardener mixed with clear poly will affect the
>amber tint, that is, the more hardener added the more amber the clear poly
>gets. Small batches make it harder to control this tinting from the chemical
>reaction. I have thought about using clear gloss polyurethane instead of
>clear poly as the topcoat if the damaged area gets little or no wear, but it
>may wear differently on edge of a lid or "age" to a different hue and be too
>noticable.  Have you tried this?

Polyurethane can be used, but it is more prone to sink out since the 
residual film thickness of polyurethane is around 35%, while 
polyester is over 70%. I prefer to use polyester if the original film 
thickness is around 10 thou or over. If the original film is very 
thin, we sometimes use acrylic lacquer. Polyurethane would be OK as 
well, but its a little harder to handle.

Lacquer is quite useful for repairing Hamburg Steinway top lid 
mouldings, which are almost universally buffed through to the wood on 
the small vertical face. We generally repair these lids with acrylic 
lacquer for the moulding only, while the top can be cut and buffed. 
When spraying a lacquer joint with original polyester, the poly 
adjacent to the joint is best lightly cut with 1200 grit paper to 
allow a key between the lacquer and polyester. If done properly, the 
joint between the polyester and lacquer is impossible to detect 
provided that the joint is right on the corner.

Well, that's it from me for a week. I'm off to experience a few days 
break with the family by the water, a couple of hours north of 
Sydney. It will be our first holiday for two years. Looking forward 
to a spot of fishing. When you have to vacuum the cobwebs off the 
fishing gear before a trip, you know its high time you got out of 
town.

Ron O.
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OVERS PIANOS - SYDNEY
Grand Piano Manufacturers

Web: http://overspianos.com.au
mailto:info@overspianos.com.au
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